package gitx

import (
	"context"
	"errors"
	"fmt"
	"sort"
	"strings"

	"github.com/go-git/go-git/v5/plumbing"
	"github.com/go-git/go-git/v5/plumbing/filemode"
	"github.com/go-git/go-git/v5/plumbing/object"
	"github.com/go-git/go-git/v5/plumbing/storer"
	"github.com/go-git/go-git/v5/storage"

	"sourcecraft.dev/bigbes/sr-ht-spec/core"
)

// Trailer is one git trailer line, "Key: Value".
//
// This package renders trailers; it does not decide which ones exist. Which
// keys are required, what an agent identity string looks like and what goes in
// X-Agent-Session are authn/'s to own — putting that policy here would give the
// git layer an opinion about identity and give the two write surfaces two
// places to drift apart. What is enforced here is only that the rendered
// message cannot be forged: a value carrying a newline could otherwise
// manufacture trailers nobody supplied.
type Trailer struct {
	Key   string
	Value string
}

func (t Trailer) validate() error {
	if t.Key == "" {
		return fmt.Errorf("gitx: trailer key is required")
	}
	for i := 0; i < len(t.Key); i++ {
		c := t.Key[i]
		ok := (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || (c >= '0' && c <= '9') || c == '-'
		if !ok {
			return fmt.Errorf("gitx: trailer key %q contains a disallowed byte %q", t.Key, c)
		}
	}
	if strings.ContainsAny(t.Value, "\n\r\x00") {
		return fmt.Errorf("gitx: trailer %q value must be a single line", t.Key)
	}
	return nil
}

// CommitMeta is everything a commit records besides its tree and parents. The
// caller supplies all of it: provenance is the product here, so nothing about
// authorship is defaulted or derived.
type CommitMeta struct {
	// Message is the commit subject and body, without a trailer block.
	Message string

	// Trailers are appended after a blank line, in order. Provenance lives here
	// rather than in a Postgres-only audit table so it is visible in plain
	// git log on any clone and cannot drift from the content it describes.
	Trailers []Trailer

	// Author is who wrote the change — for an agent commit, the agent. Committer
	// is who applied it, which is the service acting for the owner.
	Author    Signature
	Committer Signature
}

func (m CommitMeta) validate() error {
	if strings.TrimSpace(m.Message) == "" {
		return fmt.Errorf("gitx: commit message is required")
	}
	if strings.TrimSpace(strings.SplitN(m.Message, "\n", 2)[0]) == "" {
		return fmt.Errorf("gitx: commit message must open with a non-empty subject line")
	}
	if err := m.Author.validate("author"); err != nil {
		return err
	}
	if err := m.Committer.validate("committer"); err != nil {
		return err
	}
	for _, t := range m.Trailers {
		if err := t.validate(); err != nil {
			return err
		}
	}
	return nil
}

// text renders the full commit message: the body, then a blank line, then the
// trailer block, then a trailing newline.
func (m CommitMeta) text() string {
	var b strings.Builder
	b.WriteString(strings.TrimRight(strings.ReplaceAll(m.Message, "\r\n", "\n"), "\n"))
	if len(m.Trailers) > 0 {
		b.WriteString("\n\n")
		for i, t := range m.Trailers {
			if i > 0 {
				b.WriteByte('\n')
			}
			b.WriteString(t.Key)
			b.WriteString(": ")
			b.WriteString(t.Value)
		}
	}
	b.WriteByte('\n')
	return b.String()
}

// Write is a whole-document replacement at a path. There is no patch form: the
// write plane takes whole documents because that is how agents work, and it is
// what makes the merge model pure plumbing.
type Write struct {
	Path    string
	Content []byte
}

// CommitResult describes a commit this package created.
type CommitResult struct {
	Commit  plumbing.Hash
	Tree    plumbing.Hash
	Parents []plumbing.Hash
	// Blobs maps each written path to its blob sha — the render cache key for
	// the content that was just committed.
	Blobs map[string]plumbing.Hash
}

// mutableTree is a tree being built: subdirectories by name, plus the non-
// directory entries at this level. Trees are loaded whole and rewritten whole,
// which at this service's volume (tens of documents a day) costs a handful of
// tree-object reads and removes every incremental-rewrite bug class.
type mutableTree struct {
	subs  map[string]*mutableTree
	files map[string]object.TreeEntry
}

func newMutableTree() *mutableTree {
	return &mutableTree{subs: map[string]*mutableTree{}, files: map[string]object.TreeEntry{}}
}

// loadTree reads an existing tree into a mutableTree, recursively.
func (r *Repo) loadTree(t *object.Tree, depth int) (*mutableTree, error) {
	if depth > maxTreeDepth {
		return nil, fmt.Errorf("%w: tree nesting deeper than %d", ErrTooLarge, maxTreeDepth)
	}
	n := newMutableTree()
	for _, e := range t.Entries {
		if e.Mode == filemode.Dir {
			sub, err := object.GetTree(r.repo.Storer, e.Hash)
			if err != nil {
				return nil, fmt.Errorf("gitx: read tree %s in %s: %w", e.Hash, r.ref, err)
			}
			child, err := r.loadTree(sub, depth+1)
			if err != nil {
				return nil, err
			}
			n.subs[e.Name] = child
			continue
		}
		n.files[e.Name] = e
	}
	return n, nil
}

// set places a blob at path, creating intermediate trees. A component that
// collides with an existing file, or a path whose final component is an
// existing directory, is an error: silently shadowing one would replace a
// document with something that is not one.
func (n *mutableTree) set(path string, hash plumbing.Hash) error {
	comps := strings.Split(path, "/")
	cur := n
	for i, comp := range comps[:len(comps)-1] {
		if _, clash := cur.files[comp]; clash {
			return fmt.Errorf("gitx: cannot write %q: %q is a file", path, strings.Join(comps[:i+1], "/"))
		}
		next, ok := cur.subs[comp]
		if !ok {
			next = newMutableTree()
			cur.subs[comp] = next
		}
		cur = next
	}
	last := comps[len(comps)-1]
	if _, clash := cur.subs[last]; clash {
		return fmt.Errorf("gitx: cannot write %q: it is a directory", path)
	}
	cur.files[last] = object.TreeEntry{Name: last, Mode: filemode.Regular, Hash: hash}
	return nil
}

// remove deletes the blob at path if present, pruning nothing else. It reports
// whether anything was removed.
func (n *mutableTree) remove(path string) bool {
	comps := strings.Split(path, "/")
	cur := n
	for _, comp := range comps[:len(comps)-1] {
		next, ok := cur.subs[comp]
		if !ok {
			return false
		}
		cur = next
	}
	last := comps[len(comps)-1]
	if _, ok := cur.files[last]; !ok {
		return false
	}
	delete(cur.files, last)
	return true
}

// empty reports whether the tree would encode to nothing. Git has no
// representation for an empty subtree, so those are dropped on write.
func (n *mutableTree) empty() bool {
	if len(n.files) > 0 {
		return false
	}
	for _, sub := range n.subs {
		if !sub.empty() {
			return false
		}
	}
	return true
}

// write encodes the tree and every non-empty subtree, returning the root hash.
func (n *mutableTree) write(store storer.EncodedObjectStorer) (plumbing.Hash, error) {
	entries := make([]object.TreeEntry, 0, len(n.files)+len(n.subs))
	for name, e := range n.files {
		e.Name = name
		entries = append(entries, e)
	}
	for name, sub := range n.subs {
		if sub.empty() {
			continue
		}
		h, err := sub.write(store)
		if err != nil {
			return plumbing.ZeroHash, err
		}
		entries = append(entries, object.TreeEntry{Name: name, Mode: filemode.Dir, Hash: h})
	}
	// Encode refuses unsorted entries, and git compares directory names as if
	// they carried a trailing slash — TreeEntrySorter is that comparison.
	sort.Sort(object.TreeEntrySorter(entries))

	t := &object.Tree{Entries: entries}
	obj := store.NewEncodedObject()
	if err := t.Encode(obj); err != nil {
		return plumbing.ZeroHash, fmt.Errorf("gitx: encode tree: %w", err)
	}
	h, err := store.SetEncodedObject(obj)
	if err != nil {
		return plumbing.ZeroHash, fmt.Errorf("gitx: store tree: %w", err)
	}
	return h, nil
}

// writeBlob stores content as a blob, refusing anything over the document cap.
func (r *Repo) writeBlob(path string, content []byte) (plumbing.Hash, error) {
	if limit := r.blobLimit(); int64(len(content)) > limit {
		return plumbing.ZeroHash, fmt.Errorf("%w: %q is %d bytes (limit %d)",
			ErrTooLarge, path, len(content), limit)
	}
	obj := r.repo.Storer.NewEncodedObject()
	obj.SetType(plumbing.BlobObject)
	obj.SetSize(int64(len(content)))
	w, err := obj.Writer()
	if err != nil {
		return plumbing.ZeroHash, fmt.Errorf("gitx: write blob for %q: %w", path, err)
	}
	if _, err := w.Write(content); err != nil {
		w.Close()
		return plumbing.ZeroHash, fmt.Errorf("gitx: write blob for %q: %w", path, err)
	}
	if err := w.Close(); err != nil {
		return plumbing.ZeroHash, fmt.Errorf("gitx: write blob for %q: %w", path, err)
	}
	h, err := r.repo.Storer.SetEncodedObject(obj)
	if err != nil {
		return plumbing.ZeroHash, fmt.Errorf("gitx: store blob for %q: %w", path, err)
	}
	return h, nil
}

// writeCommit stores a commit object. Parents are written in the order given,
// which is load-bearing for a merge: the first parent is the approved head.
func (r *Repo) writeCommit(meta CommitMeta, tree plumbing.Hash, parents []plumbing.Hash) (plumbing.Hash, error) {
	if err := meta.validate(); err != nil {
		return plumbing.ZeroHash, err
	}
	c := &object.Commit{
		Author:       meta.Author.toGit(),
		Committer:    meta.Committer.toGit(),
		Message:      meta.text(),
		TreeHash:     tree,
		ParentHashes: parents,
	}
	obj := r.repo.Storer.NewEncodedObject()
	if err := c.Encode(obj); err != nil {
		return plumbing.ZeroHash, fmt.Errorf("gitx: encode commit: %w", err)
	}
	h, err := r.repo.Storer.SetEncodedObject(obj)
	if err != nil {
		return plumbing.ZeroHash, fmt.Errorf("gitx: store commit: %w", err)
	}
	return h, nil
}

// CreateProposalBranch cuts a new proposal branch at base.
//
// base is the agent's If-Match value: the space's approved-head sha at the time
// it read. Whether that value is still an ancestor of the approved head is the
// caller's 409 to raise (Repo.IsAncestor answers it); this function only cuts
// the branch, because the same check has to be spelled identically for REST and
// MCP and so belongs above the git layer.
func (r *Repo) CreateProposalBranch(ctx context.Context, branch, base string) (plumbing.Hash, error) {
	ctx, cancel := r.withTimeout(ctx)
	defer cancel()

	if !IsProposalBranch(branch) {
		return plumbing.ZeroHash, fmt.Errorf("%w: %q is not a %s* branch", ErrBadRev, branch, ProposalPrefix)
	}
	head, err := r.ResolveRev(ctx, base)
	if err != nil {
		return plumbing.ZeroHash, err
	}

	unlock, err := r.lock(ctx)
	if err != nil {
		return plumbing.ZeroHash, err
	}
	defer unlock()

	name := plumbing.NewBranchReferenceName(branch)
	if _, err := r.repo.Reference(name, false); err == nil {
		return plumbing.ZeroHash, fmt.Errorf("%w: branch %q in %s", ErrExists, branch, r.ref)
	} else if !errors.Is(err, plumbing.ErrReferenceNotFound) {
		return plumbing.ZeroHash, fmt.Errorf("gitx: read %s in %s: %w", name, r.ref, err)
	}
	if err := r.repo.Storer.SetReference(plumbing.NewHashReference(name, head)); err != nil {
		return plumbing.ZeroHash, fmt.Errorf("gitx: create %s in %s: %w", name, r.ref, err)
	}
	return head, nil
}

// CommitProposal commits whole-document blobs onto a proposal branch.
//
// It refuses any branch outside proposals/*: the approved branch moves in
// exactly two ways — a human push through receive-pack, or Merge — and a third
// door into it would be a way to land unreviewed agent output without a merge
// commit recording that it happened.
//
// The branch head is read, spliced and compare-and-swapped under the space
// lock, and the whole build is retried if the swap loses to a concurrent
// writer.
func (r *Repo) CommitProposal(ctx context.Context, branch string, writes []Write, meta CommitMeta) (CommitResult, error) {
	ctx, cancel := r.withTimeout(ctx)
	defer cancel()

	if !IsProposalBranch(branch) {
		return CommitResult{}, fmt.Errorf("%w: %q is not a %s* branch; only Merge writes the approved branch",
			ErrBadRev, branch, ProposalPrefix)
	}
	if len(writes) == 0 {
		return CommitResult{}, fmt.Errorf("gitx: commit to %q has no writes", branch)
	}
	if err := meta.validate(); err != nil {
		return CommitResult{}, err
	}
	seen := make(map[string]bool, len(writes))
	for _, w := range writes {
		if err := core.ValidateDocPath(w.Path); err != nil {
			return CommitResult{}, err
		}
		if seen[w.Path] {
			return CommitResult{}, fmt.Errorf("gitx: commit to %q writes %q twice", branch, w.Path)
		}
		seen[w.Path] = true
	}

	unlock, err := r.lock(ctx)
	if err != nil {
		return CommitResult{}, err
	}
	defer unlock()

	name := plumbing.NewBranchReferenceName(branch)
	var lastErr error
	for attempt := 0; attempt < r.casBudget(); attempt++ {
		if err := ctx.Err(); err != nil {
			return CommitResult{}, err
		}
		old, err := r.repo.Reference(name, false)
		if err != nil {
			return CommitResult{}, fmt.Errorf("%w: branch %q in %s: %v", ErrNotFound, branch, r.ref, err)
		}
		tree, err := r.treeOf(old.Hash())
		if err != nil {
			return CommitResult{}, err
		}
		node, err := r.loadTree(tree, 0)
		if err != nil {
			return CommitResult{}, err
		}
		blobs := make(map[string]plumbing.Hash, len(writes))
		for _, w := range writes {
			h, err := r.writeBlob(w.Path, w.Content)
			if err != nil {
				return CommitResult{}, err
			}
			if err := node.set(w.Path, h); err != nil {
				return CommitResult{}, err
			}
			blobs[w.Path] = h
		}
		treeHash, err := node.write(r.repo.Storer)
		if err != nil {
			return CommitResult{}, err
		}
		parents := []plumbing.Hash{old.Hash()}
		commit, err := r.writeCommit(meta, treeHash, parents)
		if err != nil {
			return CommitResult{}, err
		}
		r.raceHook()
		err = r.repo.Storer.CheckAndSetReference(plumbing.NewHashReference(name, commit), old)
		if err == nil {
			return CommitResult{Commit: commit, Tree: treeHash, Parents: parents, Blobs: blobs}, nil
		}
		if !errors.Is(err, storage.ErrReferenceHasChanged) {
			return CommitResult{}, fmt.Errorf("gitx: update %s in %s: %w", name, r.ref, err)
		}
		lastErr = err
	}
	return CommitResult{}, fmt.Errorf("%w: %s in %s after %d attempts: %v",
		ErrRefRace, name, r.ref, r.casBudget(), lastErr)
}