~bigbes/tarantool

tarantool-protobuf

ref: e67a90f2085f492aee03f56856200a4ed97d85d2 tarantool-protobuf/runtime/pb/c/c_runtime.c -rw-r--r-- 43.0 KiB
e67a90f2 — Eugene Blikh c_runtime: decode singular scalars (ra6 3c) 2 months ago
                                                                                
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/*
 * c_runtime.c -- C-acceleration runtime for tarantool-protobuf.
 *
 * Phase 1 (bd-mq7): descriptor -> plan compiler. Walks a finalized
 * Lua descriptor table and produces an opaque `pb_plan` userdata.
 * Encode/decode entry points arrive with bd-3b/3c/etc.; this file
 * only carries the plan compilation surface and enough introspection
 * to make a smoke test possible.
 *
 * Conventions follow docs/specs/c_accel_strategy.md.
 */

/* strdup is POSIX, not C99; declare we want the POSIX surface from
 * <string.h> before any system header pulls it in. */
#ifndef _POSIX_C_SOURCE
#define _POSIX_C_SOURCE 200809L
#endif

#include <module.h>
#include <lauxlib.h>

#include <stdint.h>
#include <stdlib.h>
#include <string.h>

#define PB_PLAN_MT      "pb.plan"
#define PB_ABI_VERSION  "1"

/* ---------------------------------------------------------------- *
 *  Kind / wire-type taxonomy.                                       *
 *                                                                  *
 *  Mirrors runtime/pb/wire.lua's TYPE_INFO. The numbering is        *
 *  internal — only the C runtime needs to agree with itself.       *
 * ---------------------------------------------------------------- */

enum {
	PB_KIND_NONE = 0,
	PB_KIND_INT32,
	PB_KIND_INT64,
	PB_KIND_UINT32,
	PB_KIND_UINT64,
	PB_KIND_SINT32,
	PB_KIND_SINT64,
	PB_KIND_FIXED32,
	PB_KIND_FIXED64,
	PB_KIND_SFIXED32,
	PB_KIND_SFIXED64,
	PB_KIND_FLOAT,
	PB_KIND_DOUBLE,
	PB_KIND_BOOL,
	PB_KIND_STRING,
	PB_KIND_BYTES,
	PB_KIND_ENUM,
	PB_KIND_MESSAGE,
	PB_KIND_MAP,
};

/* Wire types per proto3 spec. */
enum {
	PB_WIRE_VARINT = 0,
	PB_WIRE_I64    = 1,
	PB_WIRE_LEN    = 2,
	PB_WIRE_I32    = 5,
};

struct kind_info {
	const char *proto_type;
	uint8_t kind;
	uint8_t wire_type;
};

/* Lookup table keyed by proto3 field type string. Linear scan is fine —
 * a finalized descriptor calls this once per field at plan-compile time,
 * not per encode/decode. */
static const struct kind_info kind_table[] = {
	{"int32",    PB_KIND_INT32,    PB_WIRE_VARINT},
	{"int64",    PB_KIND_INT64,    PB_WIRE_VARINT},
	{"uint32",   PB_KIND_UINT32,   PB_WIRE_VARINT},
	{"uint64",   PB_KIND_UINT64,   PB_WIRE_VARINT},
	{"sint32",   PB_KIND_SINT32,   PB_WIRE_VARINT},
	{"sint64",   PB_KIND_SINT64,   PB_WIRE_VARINT},
	{"bool",     PB_KIND_BOOL,     PB_WIRE_VARINT},
	{"fixed32",  PB_KIND_FIXED32,  PB_WIRE_I32},
	{"sfixed32", PB_KIND_SFIXED32, PB_WIRE_I32},
	{"float",    PB_KIND_FLOAT,    PB_WIRE_I32},
	{"fixed64",  PB_KIND_FIXED64,  PB_WIRE_I64},
	{"sfixed64", PB_KIND_SFIXED64, PB_WIRE_I64},
	{"double",   PB_KIND_DOUBLE,   PB_WIRE_I64},
	{"string",   PB_KIND_STRING,   PB_WIRE_LEN},
	{"bytes",    PB_KIND_BYTES,    PB_WIRE_LEN},
};

static const struct kind_info *
lookup_kind(const char *proto_type)
{
	if (proto_type == NULL)
		return NULL;
	for (size_t i = 0; i < sizeof(kind_table) / sizeof(kind_table[0]); i++) {
		if (strcmp(kind_table[i].proto_type, proto_type) == 0)
			return &kind_table[i];
	}
	return NULL;
}

/* ---------------------------------------------------------------- *
 *  Plan struct layout.                                              *
 *                                                                  *
 *  Spec: docs/specs/c_accel_strategy.md § The plan userdata.        *
 *  Simplifications from the spec for mq7:                          *
 *    - oneofs[] populated but member dispatch lives in 3i          *
 *    - extension_range_* populated but dispatch lives in 3i        *
 *    - sub_plan_idx points into `sub_plans_ref` table (1-based)    *
 *    - Field name strings live in a Lua table keyed by 1..n;       *
 *      lookup via `lua_rawgeti(L, names, i+1)` per spec.           *
 * ---------------------------------------------------------------- */

typedef struct pb_plan_field {
	uint32_t field_number;
	uint8_t  wire_type;
	uint8_t  kind;
	uint8_t  packed;
	uint8_t  repeated;
	uint8_t  optional;
	uint8_t  tag_len;
	uint8_t  tag_bytes[5];
	int      sub_plan_idx;       /* 1-based into sub_plans table; 0 if none */
	uint8_t  map_key_kind;
	uint8_t  map_value_kind;
	int      map_value_sub_plan_idx; /* 1-based; 0 if value is scalar */
	int      oneof_idx;          /* 0-based into plan->oneofs; -1 if none */
	int      enum_ref;           /* LUA_REGISTRYINDEX ref for enum desc; LUA_NOREF if none */
} pb_plan_field;

typedef struct pb_plan_oneof {
	char *name;                  /* malloc'd */
	int   n_members;
	int  *member_indices;        /* indices into plan->fields */
} pb_plan_oneof;

typedef struct pb_plan {
	char *name;                  /* malloc'd descriptor name */
	int   n_fields;
	pb_plan_field *fields;
	int   n_oneofs;
	pb_plan_oneof *oneofs;
	int   extension_range_start;
	int   extension_range_end;
	uint8_t has_override;
	int   override_encode_ref;   /* LUA_NOREF if absent */
	int   override_decode_ref;
	int   field_names_ref;       /* table { [1]=name1, ... } */
	int   sub_plans_ref;         /* table { [1]=plan_userdata, ... } */
} pb_plan;

/* ---------------------------------------------------------------- *
 *  Tag encoding.                                                    *
 *                                                                  *
 *  Pre-encodes the (field_number << 3) | wire_type varint so the    *
 *  hot encode path emits a fixed memcpy instead of recomputing.    *
 *  Up to 5 bytes for any legal field number (2^29 - 1 max).        *
 * ---------------------------------------------------------------- */

static void
encode_tag(uint32_t field_number, uint8_t wire_type,
           uint8_t *out, uint8_t *out_len)
{
	uint64_t v = ((uint64_t)field_number << 3) | wire_type;
	uint8_t i = 0;
	while (v >= 0x80) {
		out[i++] = (uint8_t)(v | 0x80);
		v >>= 7;
	}
	out[i++] = (uint8_t)v;
	*out_len = i;
}

/* ---------------------------------------------------------------- *
 *  Plan lifecycle: alloc / free.                                    *
 * ---------------------------------------------------------------- */

static void
plan_free(lua_State *L, pb_plan *p)
{
	if (p->name != NULL)
		free(p->name);
	if (p->fields != NULL) {
		for (int i = 0; i < p->n_fields; i++) {
			if (p->fields[i].enum_ref != LUA_NOREF)
				luaL_unref(L, LUA_REGISTRYINDEX, p->fields[i].enum_ref);
		}
		free(p->fields);
	}
	if (p->oneofs != NULL) {
		for (int i = 0; i < p->n_oneofs; i++) {
			free(p->oneofs[i].name);
			free(p->oneofs[i].member_indices);
		}
		free(p->oneofs);
	}
	if (p->override_encode_ref != LUA_NOREF)
		luaL_unref(L, LUA_REGISTRYINDEX, p->override_encode_ref);
	if (p->override_decode_ref != LUA_NOREF)
		luaL_unref(L, LUA_REGISTRYINDEX, p->override_decode_ref);
	if (p->field_names_ref != LUA_NOREF)
		luaL_unref(L, LUA_REGISTRYINDEX, p->field_names_ref);
	if (p->sub_plans_ref != LUA_NOREF)
		luaL_unref(L, LUA_REGISTRYINDEX, p->sub_plans_ref);

	memset(p, 0, sizeof(*p));
	p->override_encode_ref = LUA_NOREF;
	p->override_decode_ref = LUA_NOREF;
	p->field_names_ref = LUA_NOREF;
	p->sub_plans_ref = LUA_NOREF;
}

static int
plan_gc(lua_State *L)
{
	pb_plan *p = (pb_plan *)luaL_checkudata(L, 1, PB_PLAN_MT);
	plan_free(L, p);
	return 0;
}

static int
plan_tostring(lua_State *L)
{
	pb_plan *p = (pb_plan *)luaL_checkudata(L, 1, PB_PLAN_MT);
	lua_pushfstring(L, "pb.plan: %s (n_fields=%d)",
	                p->name != NULL ? p->name : "(unnamed)",
	                p->n_fields);
	return 1;
}

/* ---------------------------------------------------------------- *
 *  Field compilation.                                               *
 *                                                                  *
 *  Reads one descriptor-field table (at the top of the stack) and  *
 *  fills the corresponding pb_plan_field. Recurses into sub-message *
 *  plans by calling compile_plan_impl.                              *
 *                                                                  *
 *  Stack expectations on entry:                                     *
 *    -1: field-descriptor table (e.g. {name="foo", id=1, kind=...})*
 *    sub_plans_stack_idx: the sub-plans table being filled         *
 *                                                                  *
 *  Stack on exit: same (we pop everything we push).                *
 * ---------------------------------------------------------------- */

static int compile_plan_impl(lua_State *L, int desc_idx);

/* Append a sub-plan userdata (at -1) to the sub-plans table at
 * sub_plans_idx; return its 1-based index. Pops the userdata. */
static int
push_sub_plan(lua_State *L, int sub_plans_idx)
{
	int n = (int)lua_objlen(L, sub_plans_idx);
	lua_rawseti(L, sub_plans_idx, n + 1);
	return n + 1;
}

/* Resolve a sub-message descriptor (at stack idx `sub_desc_idx`) into a
 * 1-based index into the sub-plans table. Stack-neutral: pushes/pops the
 * intermediate desc copy and plan userdata internally. */
static int
resolve_sub_plan(lua_State *L, int sub_desc_idx, int sub_plans_idx)
{
	int saved_top = lua_gettop(L);
	/* compile_plan_impl is idempotent — if desc.c_plan exists it just
	 * leaves the cached userdata on top. */
	lua_pushvalue(L, sub_desc_idx);
	int dup_idx = lua_gettop(L);
	compile_plan_impl(L, dup_idx);
	if (!lua_isuserdata(L, -1))
		luaL_error(L, "compile_plan_impl did not return a userdata");
	int idx = push_sub_plan(L, sub_plans_idx); /* pops plan userdata */
	lua_settop(L, saved_top);                  /* drop sub_desc copy */
	return idx;
}

/* Compile one field. `f_desc_idx` is the absolute stack index of the
 * field descriptor table. `field` points at the pb_plan_field slot to
 * fill. `field_names_idx`, `sub_plans_idx` are absolute indices of the
 * field-names and sub-plans tables being populated. `field_idx_1based`
 * is the 1-based position used for the field-names lookup table. */
static void
compile_field(lua_State *L, int f_desc_idx, pb_plan_field *field,
              int field_names_idx, int sub_plans_idx, int field_idx_1based)
{
	memset(field, 0, sizeof(*field));
	field->enum_ref = LUA_NOREF;
	field->oneof_idx = -1;

	/* field number */
	lua_getfield(L, f_desc_idx, "id");
	if (!lua_isnumber(L, -1))
		luaL_error(L, "field descriptor missing 'id'");
	field->field_number = (uint32_t)lua_tointeger(L, -1);
	lua_pop(L, 1);

	/* name (interned in field_names_ref under field_idx_1based) */
	lua_getfield(L, f_desc_idx, "name");
	if (!lua_isstring(L, -1))
		luaL_error(L, "field descriptor missing 'name'");
	lua_pushvalue(L, -1);                       /* dup */
	lua_rawseti(L, field_names_idx, field_idx_1based);
	lua_pop(L, 1);

	/* repeated / packed / optional */
	lua_getfield(L, f_desc_idx, "repeated");
	field->repeated = lua_toboolean(L, -1) ? 1 : 0;
	lua_pop(L, 1);
	lua_getfield(L, f_desc_idx, "packed");
	field->packed = lua_toboolean(L, -1) ? 1 : 0;
	lua_pop(L, 1);
	lua_getfield(L, f_desc_idx, "optional");
	field->optional = lua_toboolean(L, -1) ? 1 : 0;
	lua_pop(L, 1);

	/* kind dispatch on desc.kind */
	lua_getfield(L, f_desc_idx, "kind");
	const char *kind_str = lua_tostring(L, -1);
	if (kind_str == NULL)
		luaL_error(L, "field descriptor missing 'kind'");

	uint8_t element_wire_type;

	if (strcmp(kind_str, "scalar") == 0) {
		lua_pop(L, 1); /* pop kind */
		lua_getfield(L, f_desc_idx, "proto_type");
		const char *pt = lua_tostring(L, -1);
		const struct kind_info *ki = lookup_kind(pt);
		if (ki == NULL)
			luaL_error(L, "unknown scalar proto_type: %s",
			           pt != NULL ? pt : "(nil)");
		field->kind = ki->kind;
		element_wire_type = ki->wire_type;
		lua_pop(L, 1);
	} else if (strcmp(kind_str, "enum") == 0) {
		lua_pop(L, 1);
		field->kind = PB_KIND_ENUM;
		element_wire_type = PB_WIRE_VARINT;
		lua_getfield(L, f_desc_idx, "enum");
		if (lua_istable(L, -1)) {
			field->enum_ref = luaL_ref(L, LUA_REGISTRYINDEX);
		} else {
			lua_pop(L, 1);
		}
	} else if (strcmp(kind_str, "message") == 0) {
		lua_pop(L, 1);
		field->kind = PB_KIND_MESSAGE;
		element_wire_type = PB_WIRE_LEN;
		lua_getfield(L, f_desc_idx, "message");
		if (!lua_istable(L, -1))
			luaL_error(L, "message field '%s' missing 'message' descriptor",
			           "?");
		int sub_desc = lua_gettop(L);
		field->sub_plan_idx = resolve_sub_plan(L, sub_desc, sub_plans_idx);
		lua_pop(L, 1); /* sub-desc table */
	} else if (strcmp(kind_str, "map") == 0) {
		lua_pop(L, 1);
		field->kind = PB_KIND_MAP;
		element_wire_type = PB_WIRE_LEN;

		/* key */
		lua_getfield(L, f_desc_idx, "key");
		if (!lua_istable(L, -1))
			luaL_error(L, "map field missing 'key' descriptor");
		lua_getfield(L, -1, "proto_type");
		const struct kind_info *ki = lookup_kind(lua_tostring(L, -1));
		if (ki == NULL)
			luaL_error(L, "map key has unknown proto_type");
		field->map_key_kind = ki->kind;
		lua_pop(L, 2); /* proto_type + key */

		/* value */
		lua_getfield(L, f_desc_idx, "value");
		if (!lua_istable(L, -1))
			luaL_error(L, "map field missing 'value' descriptor");
		lua_getfield(L, -1, "kind");
		const char *vk = lua_tostring(L, -1);
		lua_pop(L, 1);
		if (vk != NULL && strcmp(vk, "scalar") == 0) {
			lua_getfield(L, -1, "proto_type");
			const struct kind_info *vki = lookup_kind(lua_tostring(L, -1));
			if (vki == NULL)
				luaL_error(L, "map value has unknown scalar proto_type");
			field->map_value_kind = vki->kind;
			lua_pop(L, 1);
		} else if (vk != NULL && strcmp(vk, "message") == 0) {
			field->map_value_kind = PB_KIND_MESSAGE;
			lua_getfield(L, -1, "message");
			if (lua_istable(L, -1)) {
				int sub_desc = lua_gettop(L);
				field->map_value_sub_plan_idx =
					resolve_sub_plan(L, sub_desc, sub_plans_idx);
			}
			lua_pop(L, 1);
		} else if (vk != NULL && strcmp(vk, "enum") == 0) {
			field->map_value_kind = PB_KIND_ENUM;
		} else {
			luaL_error(L, "map value has unknown kind: %s",
			           vk != NULL ? vk : "(nil)");
		}
		lua_pop(L, 1); /* value table */
	} else {
		luaL_error(L, "unknown field kind: %s", kind_str);
		return; /* unreachable */
	}

	/* Wire type: repeated+packed → LEN regardless of element type;
	 * repeated unpacked → element type per tag; singular → element. */
	if (field->repeated && field->packed) {
		field->wire_type = PB_WIRE_LEN;
	} else {
		field->wire_type = element_wire_type;
	}

	encode_tag(field->field_number, field->wire_type,
	           field->tag_bytes, &field->tag_len);
}

/* ---------------------------------------------------------------- *
 *  Oneof compilation.                                               *
 *                                                                  *
 *  desc.oneofs is the hash table {[name]=members}. We walk it and  *
 *  build pb_plan_oneof[]. For each member field, set its oneof_idx *
 *  to point back at the plan's oneof entry.                        *
 * ---------------------------------------------------------------- */

static int
find_field_by_name(pb_plan *p, lua_State *L, int field_names_idx,
                   const char *name)
{
	for (int i = 0; i < p->n_fields; i++) {
		lua_rawgeti(L, field_names_idx, i + 1);
		const char *fn = lua_tostring(L, -1);
		int match = (fn != NULL && strcmp(fn, name) == 0);
		lua_pop(L, 1);
		if (match) return i;
	}
	return -1;
}

static void
compile_oneofs(lua_State *L, pb_plan *p, int desc_idx, int field_names_idx)
{
	lua_getfield(L, desc_idx, "oneofs");
	if (!lua_istable(L, -1)) {
		lua_pop(L, 1);
		return;
	}

	/* First pass: count. */
	int count = 0;
	lua_pushnil(L);
	while (lua_next(L, -2) != 0) {
		count++;
		lua_pop(L, 1);
	}
	if (count == 0) {
		lua_pop(L, 1);
		return;
	}

	p->oneofs = (pb_plan_oneof *)calloc(count, sizeof(pb_plan_oneof));
	p->n_oneofs = count;

	int idx = 0;
	lua_pushnil(L);
	while (lua_next(L, -2) != 0) {
		/* key at -2 (oneof name), value at -1 (members array) */
		const char *oname = lua_tostring(L, -2);
		p->oneofs[idx].name = strdup(oname != NULL ? oname : "");

		int n_members = (int)lua_objlen(L, -1);
		p->oneofs[idx].n_members = n_members;
		p->oneofs[idx].member_indices = (int *)calloc(n_members, sizeof(int));

		for (int j = 0; j < n_members; j++) {
			lua_rawgeti(L, -1, j + 1);
			const char *mname = lua_tostring(L, -1);
			int fi = find_field_by_name(p, L, field_names_idx,
			                            mname != NULL ? mname : "");
			lua_pop(L, 1);
			p->oneofs[idx].member_indices[j] = fi;
			if (fi >= 0)
				p->fields[fi].oneof_idx = idx;
		}

		lua_pop(L, 1); /* value */
		idx++;
	}
	lua_pop(L, 1); /* oneofs table */
}

/* ---------------------------------------------------------------- *
 *  Main compile entry.                                              *
 *                                                                  *
 *  Idempotent. If desc.c_plan exists and is a pb.plan userdata,    *
 *  returns it. Otherwise allocates a new one, stashes it on        *
 *  desc.c_plan BEFORE recursing into sub-plans (breaks cycles for  *
 *  self-referencing messages like Person.friends → Person), then   *
 *  walks fields and oneofs.                                        *
 * ---------------------------------------------------------------- */

/* Lua 5.1 lacks lua_absindex; LuaJIT's compatibility layer too. */
static inline int
abs_idx(lua_State *L, int idx)
{
	if (idx < 0 && idx > LUA_REGISTRYINDEX)
		return lua_gettop(L) + idx + 1;
	return idx;
}

static int
compile_plan_impl(lua_State *L, int desc_idx)
{
	desc_idx = abs_idx(L, desc_idx);

	/* Idempotency check. */
	lua_getfield(L, desc_idx, "c_plan");
	if (lua_isuserdata(L, -1)) {
		/* Already compiled; leave on top of stack and pop the original
		 * desc-table push pattern wasn't done — caller still owns. */
		return 1;
	}
	lua_pop(L, 1);

	/* Allocate the plan userdata and stash it immediately. */
	pb_plan *p = (pb_plan *)lua_newuserdata(L, sizeof(pb_plan));
	memset(p, 0, sizeof(*p));
	p->override_encode_ref = LUA_NOREF;
	p->override_decode_ref = LUA_NOREF;
	p->field_names_ref = LUA_NOREF;
	p->sub_plans_ref = LUA_NOREF;

	luaL_getmetatable(L, PB_PLAN_MT);
	lua_setmetatable(L, -2);
	int plan_idx = lua_gettop(L);

	/* Stash on desc.c_plan first to break sub-message cycles. */
	lua_pushvalue(L, plan_idx);
	lua_setfield(L, desc_idx, "c_plan");

	/* desc.name */
	lua_getfield(L, desc_idx, "name");
	const char *dname = lua_tostring(L, -1);
	p->name = strdup(dname != NULL ? dname : "");
	lua_pop(L, 1);

	/* WKT override pointers — desc.encode / desc.decode. */
	lua_getfield(L, desc_idx, "encode");
	if (lua_isfunction(L, -1)) {
		p->has_override = 1;
		p->override_encode_ref = luaL_ref(L, LUA_REGISTRYINDEX);
	} else {
		lua_pop(L, 1);
	}
	lua_getfield(L, desc_idx, "decode");
	if (lua_isfunction(L, -1)) {
		p->has_override = 1;
		p->override_decode_ref = luaL_ref(L, LUA_REGISTRYINDEX);
	} else {
		lua_pop(L, 1);
	}

	/* Extension ranges (proto2). desc.extension_ranges = {{start, end}, ...} */
	lua_getfield(L, desc_idx, "extension_ranges");
	if (lua_istable(L, -1) && lua_objlen(L, -1) >= 1) {
		lua_rawgeti(L, -1, 1);
		if (lua_istable(L, -1)) {
			lua_rawgeti(L, -1, 1);
			p->extension_range_start = (int)lua_tointeger(L, -1);
			lua_pop(L, 1);
			lua_rawgeti(L, -1, 2);
			p->extension_range_end = (int)lua_tointeger(L, -1);
			lua_pop(L, 1);
		}
		lua_pop(L, 1);
	}
	lua_pop(L, 1);

	/* If this descriptor has an override, we skip field-walk entirely —
	 * the override owns encode/decode and the field array is unused. */
	if (p->has_override) {
		return 1;
	}

	/* Create the field-names and sub-plans tables. */
	lua_newtable(L);
	int field_names_idx = lua_gettop(L);
	lua_newtable(L);
	int sub_plans_idx = lua_gettop(L);

	/* Walk desc.fields. */
	lua_getfield(L, desc_idx, "fields");
	if (!lua_istable(L, -1))
		luaL_error(L, "descriptor '%s' has no 'fields'",
		           p->name != NULL ? p->name : "?");
	int n_fields = (int)lua_objlen(L, -1);
	int fields_table_idx = lua_gettop(L);

	p->n_fields = n_fields;
	p->fields = (pb_plan_field *)calloc(
		n_fields > 0 ? n_fields : 1, sizeof(pb_plan_field));

	for (int i = 0; i < n_fields; i++) {
		lua_rawgeti(L, fields_table_idx, i + 1);
		int f_desc_idx = lua_gettop(L);
		compile_field(L, f_desc_idx, &p->fields[i],
		              field_names_idx, sub_plans_idx, i + 1);
		lua_pop(L, 1);
	}

	lua_pop(L, 1); /* fields table */

	/* Compile oneofs after fields so oneof_idx back-pointers can be set. */
	compile_oneofs(L, p, desc_idx, field_names_idx);

	/* Stash the field-names + sub-plans tables in the registry. */
	lua_pushvalue(L, sub_plans_idx);
	p->sub_plans_ref = luaL_ref(L, LUA_REGISTRYINDEX);
	lua_pop(L, 1); /* sub_plans_idx (now unreferenced from stack) */

	lua_pushvalue(L, field_names_idx);
	p->field_names_ref = luaL_ref(L, LUA_REGISTRYINDEX);
	lua_pop(L, 1);

	/* Stack now has just the plan userdata on top. */
	return 1;
}

static int
compile_plan_lua(lua_State *L)
{
	luaL_checktype(L, 1, LUA_TTABLE);
	lua_settop(L, 1);
	return compile_plan_impl(L, 1);
}

/* ---------------------------------------------------------------- *
 *  Introspection (for the smoke test and future debugging).        *
 *                                                                  *
 *  These are intentionally lean — enough to let a Lua test assert  *
 *  the plan-build produced sensible values without a C-side test   *
 *  harness.                                                         *
 * ---------------------------------------------------------------- */

static int
plan_n_fields(lua_State *L)
{
	pb_plan *p = (pb_plan *)luaL_checkudata(L, 1, PB_PLAN_MT);
	lua_pushinteger(L, p->n_fields);
	return 1;
}

static int
plan_name(lua_State *L)
{
	pb_plan *p = (pb_plan *)luaL_checkudata(L, 1, PB_PLAN_MT);
	lua_pushstring(L, p->name != NULL ? p->name : "");
	return 1;
}

static int
plan_field_info(lua_State *L)
{
	pb_plan *p = (pb_plan *)luaL_checkudata(L, 1, PB_PLAN_MT);
	int i = luaL_checkint(L, 2);
	if (i < 1 || i > p->n_fields)
		return luaL_error(L, "field index %d out of range [1, %d]",
		                  i, p->n_fields);
	pb_plan_field *f = &p->fields[i - 1];

	lua_createtable(L, 0, 9);

	lua_pushinteger(L, f->field_number);
	lua_setfield(L, -2, "field_number");

	lua_pushinteger(L, f->wire_type);
	lua_setfield(L, -2, "wire_type");

	lua_pushinteger(L, f->kind);
	lua_setfield(L, -2, "kind");

	lua_pushboolean(L, f->repeated);
	lua_setfield(L, -2, "repeated");

	lua_pushboolean(L, f->packed);
	lua_setfield(L, -2, "packed");

	lua_pushboolean(L, f->optional);
	lua_setfield(L, -2, "optional");

	lua_pushinteger(L, f->sub_plan_idx);
	lua_setfield(L, -2, "sub_plan_idx");

	lua_pushinteger(L, f->oneof_idx);
	lua_setfield(L, -2, "oneof_idx");

	lua_pushlstring(L, (const char *)f->tag_bytes, f->tag_len);
	lua_setfield(L, -2, "tag_bytes");

	/* Field name from cached table. */
	lua_rawgeti(L, LUA_REGISTRYINDEX, p->field_names_ref);
	lua_rawgeti(L, -1, i);
	lua_remove(L, -2);
	lua_setfield(L, -2, "name");

	/* Map kinds, if any. */
	if (f->kind == PB_KIND_MAP) {
		lua_pushinteger(L, f->map_key_kind);
		lua_setfield(L, -2, "map_key_kind");
		lua_pushinteger(L, f->map_value_kind);
		lua_setfield(L, -2, "map_value_kind");
		lua_pushinteger(L, f->map_value_sub_plan_idx);
		lua_setfield(L, -2, "map_value_sub_plan_idx");
	}

	return 1;
}

static int
plan_n_oneofs(lua_State *L)
{
	pb_plan *p = (pb_plan *)luaL_checkudata(L, 1, PB_PLAN_MT);
	lua_pushinteger(L, p->n_oneofs);
	return 1;
}

static int
plan_oneof_info(lua_State *L)
{
	pb_plan *p = (pb_plan *)luaL_checkudata(L, 1, PB_PLAN_MT);
	int i = luaL_checkint(L, 2);
	if (i < 1 || i > p->n_oneofs)
		return luaL_error(L, "oneof index %d out of range [1, %d]",
		                  i, p->n_oneofs);
	pb_plan_oneof *o = &p->oneofs[i - 1];

	lua_createtable(L, 0, 2);
	lua_pushstring(L, o->name != NULL ? o->name : "");
	lua_setfield(L, -2, "name");

	lua_createtable(L, o->n_members, 0);
	for (int j = 0; j < o->n_members; j++) {
		lua_pushinteger(L, o->member_indices[j]);
		lua_rawseti(L, -2, j + 1);
	}
	lua_setfield(L, -2, "member_indices");

	return 1;
}

static int
plan_has_override(lua_State *L)
{
	pb_plan *p = (pb_plan *)luaL_checkudata(L, 1, PB_PLAN_MT);
	lua_pushboolean(L, p->has_override);
	return 1;
}

static int
plan_sub_plan(lua_State *L)
{
	pb_plan *p = (pb_plan *)luaL_checkudata(L, 1, PB_PLAN_MT);
	int i = luaL_checkint(L, 2);
	if (p->sub_plans_ref == LUA_NOREF || i < 1) {
		lua_pushnil(L);
		return 1;
	}
	lua_rawgeti(L, LUA_REGISTRYINDEX, p->sub_plans_ref);
	lua_rawgeti(L, -1, i);
	lua_remove(L, -2);
	return 1;
}

/* ---------------------------------------------------------------- *
 *  Encode (bd-y1n / ra6 3b): singular scalar fields.                *
 *                                                                  *
 *  Repeated/packed, map, and message-typed fields are skipped —    *
 *  those land in 3d (sub-message) and 3e (repeated/packed).         *
 *                                                                  *
 *  Buffer strategy: a 4KB stack-backed scratch buffer that promotes *
 *  to a Lua userdata (GC'd automatically) on overflow. Using        *
 *  `lua_newuserdata` for heap growth means a luaL_error mid-encode  *
 *  doesn't leak — the userdata is still on the stack at the unwind  *
 *  point and gets collected normally.                               *
 * ---------------------------------------------------------------- */

#define ENC_STACK_BUF 4096

typedef struct enc_buf {
	uint8_t  stack[ENC_STACK_BUF];
	uint8_t *heap;       /* pointer into Lua userdata when grown; NULL while on stack */
	int      heap_idx;   /* stack slot of the userdata; 0 if not yet on heap */
	size_t   cap;
	size_t   used;
} enc_buf;

static inline uint8_t *
ebuf_base(enc_buf *b)
{
	return b->heap != NULL ? b->heap : b->stack;
}

static void
ebuf_init(enc_buf *b)
{
	b->heap = NULL;
	b->heap_idx = 0;
	b->cap = ENC_STACK_BUF;
	b->used = 0;
}

static void
ebuf_grow(lua_State *L, enc_buf *b, size_t needed)
{
	size_t new_cap = b->cap * 2;
	while (new_cap - b->used < needed)
		new_cap *= 2;

	uint8_t *new_buf = (uint8_t *)lua_newuserdata(L, new_cap);
	memcpy(new_buf, ebuf_base(b), b->used);
	if (b->heap_idx == 0) {
		b->heap_idx = lua_gettop(L);
	} else {
		lua_replace(L, b->heap_idx);
	}
	b->heap = new_buf;
	b->cap = new_cap;
}

static inline void
ebuf_reserve(lua_State *L, enc_buf *b, size_t needed)
{
	if (b->cap - b->used < needed)
		ebuf_grow(L, b, needed);
}

static inline void
ebuf_put_byte(enc_buf *b, uint8_t v)
{
	ebuf_base(b)[b->used++] = v;
}

static inline void
ebuf_put_bytes(enc_buf *b, const uint8_t *src, size_t n)
{
	memcpy(ebuf_base(b) + b->used, src, n);
	b->used += n;
}

static inline void
ebuf_put_varint(enc_buf *b, uint64_t v)
{
	uint8_t *p = ebuf_base(b) + b->used;
	while (v >= 0x80) {
		*p++ = (uint8_t)(v | 0x80);
		v >>= 7;
	}
	*p++ = (uint8_t)v;
	b->used = (size_t)(p - ebuf_base(b));
}

static inline void
ebuf_put_fixed32(enc_buf *b, uint32_t v)
{
	uint8_t *p = ebuf_base(b) + b->used;
	p[0] = (uint8_t)v;
	p[1] = (uint8_t)(v >> 8);
	p[2] = (uint8_t)(v >> 16);
	p[3] = (uint8_t)(v >> 24);
	b->used += 4;
}

static inline void
ebuf_put_fixed64(enc_buf *b, uint64_t v)
{
	uint8_t *p = ebuf_base(b) + b->used;
	p[0] = (uint8_t)v;
	p[1] = (uint8_t)(v >> 8);
	p[2] = (uint8_t)(v >> 16);
	p[3] = (uint8_t)(v >> 24);
	p[4] = (uint8_t)(v >> 32);
	p[5] = (uint8_t)(v >> 40);
	p[6] = (uint8_t)(v >> 48);
	p[7] = (uint8_t)(v >> 56);
	b->used += 8;
}

static inline void
ebuf_put_tag(enc_buf *b, const pb_plan_field *f)
{
	memcpy(ebuf_base(b) + b->used, f->tag_bytes, f->tag_len);
	b->used += f->tag_len;
}

/* Read a Lua value as uint64. Mirrors wire.lua's to_uint64: negative
 * Lua numbers are sign-extended through int64 (proto3 wire spec for
 * int32 fields). int64/uint64 cdata flow through luaL_touint64. */
static uint64_t
to_uint64_at(lua_State *L, int idx)
{
	if (lua_type(L, idx) == LUA_TNUMBER) {
		double d = lua_tonumber(L, idx);
		if (d < 0)
			return (uint64_t)(int64_t)d;
		return (uint64_t)d;
	}
	return luaL_touint64(L, idx);
}

static int32_t
to_int32_at(lua_State *L, int idx)
{
	if (lua_type(L, idx) == LUA_TNUMBER)
		return (int32_t)lua_tointeger(L, idx);
	return (int32_t)luaL_toint64(L, idx);
}

static int64_t
to_int64_at(lua_State *L, int idx)
{
	if (lua_type(L, idx) == LUA_TNUMBER)
		return (int64_t)lua_tonumber(L, idx);
	return luaL_toint64(L, idx);
}

static inline uint32_t
zigzag32(int32_t n)
{
	return ((uint32_t)n << 1) ^ (uint32_t)(n >> 31);
}

static inline uint64_t
zigzag64(int64_t n)
{
	return ((uint64_t)n << 1) ^ (uint64_t)(n >> 63);
}

static inline uint32_t
f32_to_u32(float f)
{
	union { float f; uint32_t u; } pun;
	pun.f = f;
	return pun.u;
}

static inline uint64_t
f64_to_u64(double d)
{
	union { double d; uint64_t u; } pun;
	pun.d = d;
	return pun.u;
}

/* Resolve an enum field's Lua value to its int32 ordinal. Accepts
 * numbers (returned directly), cdata int64 (downcast), or strings
 * (looked up in field->enum_ref's by_name table). Mirrors the
 * `if type(v) == 'string' then nv = M.Status[v]` branch in mode=full. */
static int32_t
enum_value_at(lua_State *L, pb_plan_field *f, int idx)
{
	int t = lua_type(L, idx);
	if (t == LUA_TSTRING) {
		const char *s = lua_tostring(L, idx);
		if (f->enum_ref == LUA_NOREF)
			luaL_error(L, "enum field '%s' has no enum descriptor", s);
		lua_rawgeti(L, LUA_REGISTRYINDEX, f->enum_ref);
		lua_getfield(L, -1, "by_name");
		if (!lua_istable(L, -1)) {
			luaL_error(L, "enum descriptor missing by_name");
		}
		lua_pushvalue(L, idx);
		lua_rawget(L, -2);
		if (lua_isnil(L, -1)) {
			luaL_error(L, "unknown enum value '%s'", s);
		}
		int32_t v = (int32_t)lua_tointeger(L, -1);
		lua_pop(L, 3); /* value + by_name + desc */
		return v;
	}
	return (int32_t)to_int64_at(L, idx);
}

/* Encode one singular scalar/enum/string field. Returns 1 if bytes
 * were written, 0 if the value collapsed to its proto3 default and
 * was suppressed. Zero-suppression skipped for proto3-optional. */
static int
encode_one_field(lua_State *L, enc_buf *b, pb_plan_field *f, int val_idx)
{
	switch (f->kind) {
	case PB_KIND_INT32:
	case PB_KIND_INT64:
	case PB_KIND_UINT32:
	case PB_KIND_UINT64: {
		uint64_t u = to_uint64_at(L, val_idx);
		if (!f->optional && u == 0) return 0;
		ebuf_reserve(L, b, f->tag_len + 10);
		ebuf_put_tag(b, f);
		ebuf_put_varint(b, u);
		return 1;
	}
	case PB_KIND_SINT32: {
		int32_t s = to_int32_at(L, val_idx);
		if (!f->optional && s == 0) return 0;
		ebuf_reserve(L, b, f->tag_len + 5);
		ebuf_put_tag(b, f);
		ebuf_put_varint(b, zigzag32(s));
		return 1;
	}
	case PB_KIND_SINT64: {
		int64_t s = to_int64_at(L, val_idx);
		if (!f->optional && s == 0) return 0;
		ebuf_reserve(L, b, f->tag_len + 10);
		ebuf_put_tag(b, f);
		ebuf_put_varint(b, zigzag64(s));
		return 1;
	}
	case PB_KIND_BOOL: {
		int truthy = lua_toboolean(L, val_idx);
		if (!f->optional && !truthy) return 0;
		ebuf_reserve(L, b, f->tag_len + 1);
		ebuf_put_tag(b, f);
		ebuf_put_byte(b, truthy ? 1 : 0);
		return 1;
	}
	case PB_KIND_FIXED32:
	case PB_KIND_SFIXED32: {
		uint64_t u = to_uint64_at(L, val_idx);
		uint32_t u32 = (uint32_t)u;
		if (!f->optional && u32 == 0) return 0;
		ebuf_reserve(L, b, f->tag_len + 4);
		ebuf_put_tag(b, f);
		ebuf_put_fixed32(b, u32);
		return 1;
	}
	case PB_KIND_FIXED64:
	case PB_KIND_SFIXED64: {
		uint64_t u = to_uint64_at(L, val_idx);
		if (!f->optional && u == 0) return 0;
		ebuf_reserve(L, b, f->tag_len + 8);
		ebuf_put_tag(b, f);
		ebuf_put_fixed64(b, u);
		return 1;
	}
	case PB_KIND_FLOAT: {
		double d = lua_tonumber(L, val_idx);
		uint32_t u = f32_to_u32((float)d);
		/* +0.0 -> u==0 (skip); -0.0 -> u==0x80000000 (emit). Matches the
		 * Lua-side `(v ~= 0 or 1/v == -math.huge)` guard. */
		if (!f->optional && u == 0) return 0;
		ebuf_reserve(L, b, f->tag_len + 4);
		ebuf_put_tag(b, f);
		ebuf_put_fixed32(b, u);
		return 1;
	}
	case PB_KIND_DOUBLE: {
		double d = lua_tonumber(L, val_idx);
		uint64_t u = f64_to_u64(d);
		if (!f->optional && u == 0) return 0;
		ebuf_reserve(L, b, f->tag_len + 8);
		ebuf_put_tag(b, f);
		ebuf_put_fixed64(b, u);
		return 1;
	}
	case PB_KIND_ENUM: {
		int32_t e = enum_value_at(L, f, val_idx);
		if (!f->optional && e == 0) return 0;
		ebuf_reserve(L, b, f->tag_len + 10);
		ebuf_put_tag(b, f);
		/* enum is wire-equivalent to int32: sign-extend to uint64 then varint. */
		ebuf_put_varint(b, (uint64_t)(int64_t)e);
		return 1;
	}
	case PB_KIND_STRING:
	case PB_KIND_BYTES: {
		if (lua_type(L, val_idx) != LUA_TSTRING)
			luaL_error(L, "string/bytes field requires a string value");
		size_t len;
		const char *s = lua_tolstring(L, val_idx, &len);
		if (!f->optional && len == 0) return 0;
		ebuf_reserve(L, b, f->tag_len + 10 + len);
		ebuf_put_tag(b, f);
		ebuf_put_varint(b, (uint64_t)len);
		if (len > 0)
			ebuf_put_bytes(b, (const uint8_t *)s, len);
		return 1;
	}
	default:
		/* Repeated/map/message are 3d/3e scope — silently skip. */
		return 0;
	}
}

static int
encode_lua(lua_State *L)
{
	pb_plan *plan = (pb_plan *)luaL_checkudata(L, 1, PB_PLAN_MT);
	luaL_checktype(L, 2, LUA_TTABLE);
	int msg_idx = 2;

	if (plan->has_override) {
		return luaL_error(L,
			"C encode on plan with override is unsupported (bd-rmf)");
	}

	/* Pin the field-names table on the stack for the loop's rawgeti. */
	if (plan->field_names_ref == LUA_NOREF || plan->n_fields == 0) {
		lua_pushliteral(L, "");
		return 1;
	}
	lua_rawgeti(L, LUA_REGISTRYINDEX, plan->field_names_ref);
	int names_idx = lua_gettop(L);

	enc_buf b;
	ebuf_init(&b);

	for (int i = 0; i < plan->n_fields; i++) {
		pb_plan_field *f = &plan->fields[i];

		/* 3b scope: singular scalar/enum/string/bytes. Repeated, map,
		 * and message-typed fields are silently skipped — they land
		 * in 3d/3e and will overwrite this short-circuit. */
		if (f->repeated) continue;
		if (f->kind == PB_KIND_MAP || f->kind == PB_KIND_MESSAGE)
			continue;

		lua_rawgeti(L, names_idx, i + 1); /* push field name */
		lua_rawget(L, msg_idx);            /* push msg[name] */
		int val_idx = lua_gettop(L);

		if (lua_isnil(L, val_idx)) {
			lua_pop(L, 1);
			continue;
		}

		encode_one_field(L, &b, f, val_idx);
		lua_pop(L, 1);
	}

	lua_pushlstring(L, (const char *)ebuf_base(&b), b.used);
	return 1;
}

/* ---------------------------------------------------------------- *
 *  Decode (bd-mz6 / ra6 3c): singular scalar fields.                *
 *                                                                  *
 *  Symmetric scope with 3b. Repeated/map/message-typed fields are  *
 *  skipped over by wire-type — 3d/3e/3i will overwrite this with   *
 *  real handling. Unknown tags are skipped silently too; bd-wyp    *
 *  will add round-trip capture.                                     *
 *                                                                  *
 *  Result-table shape must match mode=full pure-Lua decode. That   *
 *  pins per-kind value types: int64/uint64/sint64/fixed64/sfixed64 *
 *  push Tarantool int64_t/uint64_t cdata via luaL_pushint64 /      *
 *  luaL_pushuint64; everything else pushes a Lua number, boolean,  *
 *  or string. See runtime/pb/wire.lua decoder comments.             *
 * ---------------------------------------------------------------- */

typedef struct dec_ctx {
	lua_State    *L;
	const uint8_t *buf;
	size_t        len;
	size_t        pos;
} dec_ctx;

static uint64_t
dec_varint(dec_ctx *c)
{
	uint64_t v = 0;
	int shift = 0;
	for (int i = 0; i < 10; i++) {
		if (c->pos >= c->len)
			luaL_error(c->L, "truncated varint at offset %d",
			           (int)c->pos);
		uint8_t b = c->buf[c->pos++];
		v |= ((uint64_t)(b & 0x7f)) << shift;
		if ((b & 0x80) == 0)
			return v;
		shift += 7;
	}
	luaL_error(c->L, "varint exceeds 10 bytes at offset %d",
	           (int)c->pos);
	return 0;
}

static uint32_t
dec_fixed32(dec_ctx *c)
{
	if (c->len - c->pos < 4)
		luaL_error(c->L, "truncated fixed32 at offset %d",
		           (int)c->pos);
	uint32_t v = (uint32_t)c->buf[c->pos]
	           | ((uint32_t)c->buf[c->pos + 1] << 8)
	           | ((uint32_t)c->buf[c->pos + 2] << 16)
	           | ((uint32_t)c->buf[c->pos + 3] << 24);
	c->pos += 4;
	return v;
}

static uint64_t
dec_fixed64(dec_ctx *c)
{
	if (c->len - c->pos < 8)
		luaL_error(c->L, "truncated fixed64 at offset %d",
		           (int)c->pos);
	uint64_t v = 0;
	for (int i = 0; i < 8; i++)
		v |= ((uint64_t)c->buf[c->pos + i]) << (i * 8);
	c->pos += 8;
	return v;
}

static void
dec_skip(dec_ctx *c, uint8_t wt)
{
	switch (wt) {
	case PB_WIRE_VARINT:
		(void)dec_varint(c);
		break;
	case PB_WIRE_I32:
		(void)dec_fixed32(c);
		break;
	case PB_WIRE_I64:
		(void)dec_fixed64(c);
		break;
	case PB_WIRE_LEN: {
		uint64_t plen = dec_varint(c);
		if (c->len - c->pos < plen)
			luaL_error(c->L, "truncated length-delimited payload");
		c->pos += (size_t)plen;
		break;
	}
	default:
		luaL_error(c->L, "unsupported wire type %d for skip", (int)wt);
	}
}

static inline int32_t
zigzag32_dec(uint32_t u)
{
	return (int32_t)((u >> 1) ^ (~(u & 1) + 1));
}

static inline int64_t
zigzag64_dec(uint64_t u)
{
	return (int64_t)((u >> 1) ^ (~(u & 1) + 1));
}

/* Decode a single value for field `f` and push it onto the Lua stack. */
static void
dec_push_one(dec_ctx *c, pb_plan_field *f)
{
	switch (f->kind) {
	case PB_KIND_INT32: {
		uint64_t u = dec_varint(c);
		int32_t v = (int32_t)(uint32_t)u;
		lua_pushinteger(c->L, v);
		return;
	}
	case PB_KIND_INT64: {
		uint64_t u = dec_varint(c);
		luaL_pushint64(c->L, (int64_t)u);
		return;
	}
	case PB_KIND_UINT32: {
		uint64_t u = dec_varint(c);
		uint32_t v = (uint32_t)u;
		lua_pushnumber(c->L, (double)v);
		return;
	}
	case PB_KIND_UINT64: {
		uint64_t u = dec_varint(c);
		luaL_pushuint64(c->L, u);
		return;
	}
	case PB_KIND_SINT32: {
		uint64_t u = dec_varint(c);
		int32_t v = zigzag32_dec((uint32_t)u);
		lua_pushinteger(c->L, v);
		return;
	}
	case PB_KIND_SINT64: {
		uint64_t u = dec_varint(c);
		luaL_pushint64(c->L, zigzag64_dec(u));
		return;
	}
	case PB_KIND_BOOL: {
		uint64_t u = dec_varint(c);
		lua_pushboolean(c->L, u != 0);
		return;
	}
	case PB_KIND_ENUM: {
		uint64_t u = dec_varint(c);
		int32_t v = (int32_t)(uint32_t)u;
		lua_pushinteger(c->L, v);
		return;
	}
	case PB_KIND_FIXED32: {
		uint32_t u = dec_fixed32(c);
		lua_pushnumber(c->L, (double)u);
		return;
	}
	case PB_KIND_SFIXED32: {
		uint32_t u = dec_fixed32(c);
		lua_pushinteger(c->L, (int32_t)u);
		return;
	}
	case PB_KIND_FLOAT: {
		uint32_t u = dec_fixed32(c);
		union { uint32_t u; float f; } pun;
		pun.u = u;
		lua_pushnumber(c->L, (double)pun.f);
		return;
	}
	case PB_KIND_FIXED64: {
		uint64_t u = dec_fixed64(c);
		luaL_pushuint64(c->L, u);
		return;
	}
	case PB_KIND_SFIXED64: {
		uint64_t u = dec_fixed64(c);
		luaL_pushint64(c->L, (int64_t)u);
		return;
	}
	case PB_KIND_DOUBLE: {
		uint64_t u = dec_fixed64(c);
		union { uint64_t u; double d; } pun;
		pun.u = u;
		lua_pushnumber(c->L, pun.d);
		return;
	}
	case PB_KIND_STRING:
	case PB_KIND_BYTES: {
		uint64_t plen = dec_varint(c);
		if (c->len - c->pos < plen)
			luaL_error(c->L, "truncated string/bytes payload");
		lua_pushlstring(c->L, (const char *)(c->buf + c->pos),
		                (size_t)plen);
		c->pos += (size_t)plen;
		return;
	}
	default:
		luaL_error(c->L, "dec_push_one: unsupported kind %d",
		           (int)f->kind);
	}
}

static int
decode_lua(lua_State *L)
{
	pb_plan *plan = (pb_plan *)luaL_checkudata(L, 1, PB_PLAN_MT);
	size_t buf_len;
	const char *buf = luaL_checklstring(L, 2, &buf_len);

	if (plan->has_override) {
		return luaL_error(L,
			"C decode on plan with override is unsupported (bd-rmf)");
	}

	/* Pre-size the result table: at most n_fields singular entries. */
	lua_createtable(L, 0, plan->n_fields);
	int result_idx = lua_gettop(L);

	/* Pin field-names so the loop can rawgeti by 1-based field index. */
	if (plan->field_names_ref != LUA_NOREF)
		lua_rawgeti(L, LUA_REGISTRYINDEX, plan->field_names_ref);
	else
		lua_pushnil(L);
	int names_idx = lua_gettop(L);

	dec_ctx c;
	c.L   = L;
	c.buf = (const uint8_t *)buf;
	c.len = buf_len;
	c.pos = 0;

	while (c.pos < c.len) {
		uint64_t tag = dec_varint(&c);
		uint32_t field_number = (uint32_t)(tag >> 3);
		uint8_t  wt           = (uint8_t)(tag & 0x07);

		/* Linear scan over plan->fields. n_fields is typically small;
		 * tag-keyed dispatch table is a future optimization. */
		pb_plan_field *f = NULL;
		for (int i = 0; i < plan->n_fields; i++) {
			if (plan->fields[i].field_number == field_number) {
				f = &plan->fields[i];
				break;
			}
		}

		/* Out-of-scope shapes for 3c: repeated, map, message. Skip
		 * by wire type so 3d/3e/3i can replace this branch later. */
		if (f == NULL
		    || f->repeated
		    || f->kind == PB_KIND_MAP
		    || f->kind == PB_KIND_MESSAGE) {
			dec_skip(&c, wt);
			continue;
		}

		dec_push_one(&c, f);  /* stack: ..., names, value */

		/* result[name] = value. Field index within plan = pointer
		 * arithmetic against plan->fields[0]; +1 for the 1-based
		 * field_names_ref convention used elsewhere. */
		int field_idx_1based = (int)(f - plan->fields) + 1;
		lua_rawgeti(L, names_idx, field_idx_1based);  /* push name */
		lua_insert(L, -2);                            /* name, value */
		lua_rawset(L, result_idx);                    /* result[name] = value */
	}

	/* Drop names ref; leave result on top. */
	lua_pop(L, 1);
	return 1;
}

/* ---------------------------------------------------------------- *
 *  Module entry.                                                    *
 * ---------------------------------------------------------------- */

static const struct luaL_Reg c_runtime_methods[] = {
	{"compile_plan",    compile_plan_lua},
	{"plan_n_fields",   plan_n_fields},
	{"plan_name",       plan_name},
	{"plan_field_info", plan_field_info},
	{"plan_n_oneofs",   plan_n_oneofs},
	{"plan_oneof_info", plan_oneof_info},
	{"plan_has_override", plan_has_override},
	{"plan_sub_plan",   plan_sub_plan},
	{"encode",          encode_lua},
	{"decode",          decode_lua},
	{NULL, NULL},
};

static const struct luaL_Reg plan_mt_methods[] = {
	{"__gc",       plan_gc},
	{"__tostring", plan_tostring},
	{NULL, NULL},
};

LUA_API int
luaopen_pb_c_runtime(lua_State *L)
{
	/* Register the plan metatable. */
	luaL_newmetatable(L, PB_PLAN_MT);
	luaL_register(L, NULL, plan_mt_methods);
	lua_pop(L, 1);

	/* Build the module table. */
	lua_newtable(L);
	luaL_register(L, NULL, c_runtime_methods);

	lua_pushliteral(L, PB_ABI_VERSION);
	lua_setfield(L, -2, "_abi_version");

	/* Kind constants — exported so Lua tests can compare without
	 * duplicating the enum. */
	lua_createtable(L, 0, 19);
	lua_pushinteger(L, PB_KIND_NONE);     lua_setfield(L, -2, "NONE");
	lua_pushinteger(L, PB_KIND_INT32);    lua_setfield(L, -2, "INT32");
	lua_pushinteger(L, PB_KIND_INT64);    lua_setfield(L, -2, "INT64");
	lua_pushinteger(L, PB_KIND_UINT32);   lua_setfield(L, -2, "UINT32");
	lua_pushinteger(L, PB_KIND_UINT64);   lua_setfield(L, -2, "UINT64");
	lua_pushinteger(L, PB_KIND_SINT32);   lua_setfield(L, -2, "SINT32");
	lua_pushinteger(L, PB_KIND_SINT64);   lua_setfield(L, -2, "SINT64");
	lua_pushinteger(L, PB_KIND_FIXED32);  lua_setfield(L, -2, "FIXED32");
	lua_pushinteger(L, PB_KIND_FIXED64);  lua_setfield(L, -2, "FIXED64");
	lua_pushinteger(L, PB_KIND_SFIXED32); lua_setfield(L, -2, "SFIXED32");
	lua_pushinteger(L, PB_KIND_SFIXED64); lua_setfield(L, -2, "SFIXED64");
	lua_pushinteger(L, PB_KIND_FLOAT);    lua_setfield(L, -2, "FLOAT");
	lua_pushinteger(L, PB_KIND_DOUBLE);   lua_setfield(L, -2, "DOUBLE");
	lua_pushinteger(L, PB_KIND_BOOL);     lua_setfield(L, -2, "BOOL");
	lua_pushinteger(L, PB_KIND_STRING);   lua_setfield(L, -2, "STRING");
	lua_pushinteger(L, PB_KIND_BYTES);    lua_setfield(L, -2, "BYTES");
	lua_pushinteger(L, PB_KIND_ENUM);     lua_setfield(L, -2, "ENUM");
	lua_pushinteger(L, PB_KIND_MESSAGE);  lua_setfield(L, -2, "MESSAGE");
	lua_pushinteger(L, PB_KIND_MAP);      lua_setfield(L, -2, "MAP");
	lua_setfield(L, -2, "KIND");

	lua_createtable(L, 0, 4);
	lua_pushinteger(L, PB_WIRE_VARINT); lua_setfield(L, -2, "VARINT");
	lua_pushinteger(L, PB_WIRE_I64);    lua_setfield(L, -2, "I64");
	lua_pushinteger(L, PB_WIRE_LEN);    lua_setfield(L, -2, "LEN");
	lua_pushinteger(L, PB_WIRE_I32);    lua_setfield(L, -2, "I32");
	lua_setfield(L, -2, "WIRE");

	return 1;
}