package web

import (
	"bytes"
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"html/template"
	"log/slog"
	"strings"

	"go.bigb.es/auxilia/scribe"

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

// inlineSimilarityThreshold is the Phase 0 verdict's presentation switch: a
// modified prose block whose token similarity is at or above it renders as an
// inline word diff, and one below it renders as a paired old/new region.
//
// 13% of real prose modifications shred into interleaved fragments — those
// paragraphs really were rewritten sentence by sentence — and every one of them
// scores at or below 0.73. Rendering them inline makes one review in eight
// unreadable, which is the one where the agent changed the most. prosediff
// exports BlockChange.Similarity for exactly this decision and computes no HTML
// itself; this is where the decision is made.
const inlineSimilarityThreshold = 0.75

// diffView is the whole rendered diff of one document, ready for the proposal
// template. Unchanged reports the degenerate case — a proposal that touches a
// document without changing it — so the page can say so rather than show an
// empty diff.
type diffView struct {
	HTML      template.HTML
	Stats     prosediff.Stats
	Unchanged bool

	// Unplaced are this document's threads that no rendered block claimed. The
	// page shows them in its own area: a comment whose anchor is lost, or whose
	// block this diff does not render, must still be visible somewhere.
	Unplaced []service.Thread
}

// docDiff is one document's review: the two revisions to compare, the identity
// its comments anchor to, the threads already resolved against this revision,
// and who may act on them.
type docDiff struct {
	// DocID is the document's anchoring key — see docIDFor.
	DocID string
	// Path is the document's path on the proposal branch, which the compose form
	// posts back so the anchor is rebuilt against the branch, not the form.
	Path string
	// Base is the approved content the proposal was made against; Proposed is
	// what the branch says now.
	Base, Proposed []byte
	// Threads are this document's threads, already run through
	// service.AnchorThreads: State and Block are meaningless before that.
	Threads  []service.Thread
	Controls reviewControls
}

// blockKey identifies a block of one revision of one document: which side it is
// on and its position in that side's segmentation. It is the only key a thread
// is placed by — see docRenderer.
type blockKey struct {
	side    core.CommentSide
	ordinal int
}

// renderDocDiff diffs the approved (old) and proposed (new) source of one
// document and renders it as a line-numbered unified diff, with every block's
// review threads and, for the owner, the form that opens a new one.
//
// The page is a table because the gutter has to be a gutter: two number tracks
// that stay aligned with the first visual line of a prose line that wraps three
// times. Selection is by line and anchoring is by block, so every row carries
// the anchor of the block it belongs to and the block's first row carries the id
// a comment link scrolls to.
//
// The arithmetic — which number belongs in which track, and which lines a fold
// may hide — is diffrows.go's, deliberately kept out of here. What is left is
// escaping and concatenation: every piece of document content passes through
// template.HTMLEscapeString, and the only markup this produces is its own
// structure plus the thread markup html/template escapes for it.
func renderDocDiff(in docDiff) diffView {
	d := prosediff.Compare(in.Base, in.Proposed)
	view := diffView{Stats: d.Stats, Unchanged: !d.Stats.Changed()}

	r := newDocRenderer(in, d)
	if view.Unchanged {
		// Nothing is rendered, so nothing can hold a thread. Handing them all
		// back keeps the invariant this renderer is built on: every thread comes
		// out either attached to a block or in Unplaced, and never neither.
		view.Unplaced = r.unplaced()
		return view
	}

	var b strings.Builder
	b.WriteString(`<div class="prosediff"><table class="ph-diff">`)
	for _, g := range groupRows(buildRows(d.Changes, r.blockInfo)) {
		r.writeGroup(&b, g)
	}
	b.WriteString(`</table></div>`)

	view.HTML = template.HTML(b.String())
	view.Unplaced = r.unplaced()
	return view
}

// docRenderer renders the rows of one document's diff. It holds the anchor
// numbering of both revisions and the threads still waiting for a block: a
// block claims its threads as its notes row is written, and whatever is left
// over at the end never had a block on the page.
type docRenderer struct {
	in docDiff
	// anchors is each side's blocks, numbered within their heading path.
	anchors map[core.CommentSide][]core.AnchorBlock
	pending map[blockKey][]service.Thread
	// foldPrefix scopes this document's fold checkbox ids. A proposal page
	// renders several documents into one HTML document, and two folds sharing an
	// id would toggle each other.
	foldPrefix string
}

func newDocRenderer(in docDiff, d *prosediff.Diff) *docRenderer {
	sum := sha256.Sum256([]byte(in.Path))
	r := &docRenderer{
		in: in,
		anchors: map[core.CommentSide][]core.AnchorBlock{
			core.SideNew: blockAnchors(d.NewBlocks),
			core.SideOld: blockAnchors(d.OldBlocks),
		},
		pending:    make(map[blockKey][]service.Thread, len(in.Threads)),
		foldPrefix: hex.EncodeToString(sum[:])[:8],
	}
	// A thread is placed by (side, block ordinal) and by nothing else. The
	// anchor resolution already decided which block it belongs to, and any
	// second-guessing here would be the one thing the anchor model forbids: a
	// comment quietly moved onto a neighbouring paragraph. A thread whose anchor
	// did not resolve (Block < 0) is never placed at all.
	for _, t := range in.Threads {
		if t.Block < 0 {
			continue
		}
		k := blockKey{sideOf(t.Anchor.Side), t.Block}
		r.pending[k] = append(r.pending[k], t)
	}
	return r
}

// unplaced reports the threads no block claimed. It walks the input rather than
// the leftover map so the order is the one the service listed them in, not a
// map's.
func (r *docRenderer) unplaced() []service.Thread {
	out := make([]service.Thread, 0, len(r.pending))
	for _, t := range r.in.Threads {
		if t.Block < 0 {
			out = append(out, t)
			continue
		}
		k := blockKey{sideOf(t.Anchor.Side), t.Block}
		if _, still := r.pending[k]; still {
			out = append(out, t)
		}
	}
	return out
}

// target returns the anchor of the block a change offers to comment on, and
// whether it offers one at all.
//
// A comment goes on the new side, which is the text under review; the old side
// is for a block the proposal deletes, where there is no new text to point at.
// A move-out offers nothing: it is a pointer to text that is rendered at its
// new position, and anchoring it here would give one paragraph two places to be
// commented on.
func (r *docRenderer) target(c prosediff.BlockChange) (core.CommentAnchor, blockKey, bool) {
	var side core.CommentSide
	var blk *prosediff.Block
	switch c.Kind {
	case prosediff.ChangeMoveOut:
		return core.CommentAnchor{}, blockKey{}, false
	case prosediff.ChangeDelete:
		side, blk = core.SideOld, c.Old
	default:
		side, blk = core.SideNew, c.New
	}
	blocks := r.anchors[side]
	if blk == nil || blk.Ordinal < 0 || blk.Ordinal >= len(blocks) {
		return core.CommentAnchor{}, blockKey{}, false
	}
	ab := blocks[blk.Ordinal]
	anchor := core.CommentAnchor{
		DocID:       r.in.DocID,
		HeadingPath: ab.HeadingPath,
		Index:       ab.Index,
		BlockHash:   ab.Hash,
		Side:        side,
	}
	return anchor, blockKey{side, blk.Ordinal}, true
}

// blockInfo answers, for the row builder, the two questions about a change that
// are not in the change: what it anchors to, and whether it needs a notes row.
//
// It counts this block's pending threads without claiming them — claiming
// happens when the notes row is written, which is the one place that can
// guarantee they were actually rendered. A block the anchor numbering does not
// cover is still given rows, because losing document content would be worse
// than losing its comment affordance; it just carries no id and offers no form.
func (r *docRenderer) blockInfo(c prosediff.BlockChange) blockInfo {
	anchor, key, ok := r.target(c)
	if !ok {
		return blockInfo{}
	}
	threads := len(r.pending[key]) > 0
	return blockInfo{
		Anchor:      anchor,
		Key:         key,
		Commentable: true,
		HasThreads:  threads,
		Notes:       threads || r.in.Controls.Owner,
	}
}

// writeGroup renders one <tbody>. A fold group opens with the checkbox and
// label that reveal it: a real form control rather than a script-driven button,
// so an unchanged run can be opened with JavaScript off.
func (r *docRenderer) writeGroup(b *strings.Builder, g rowGroup) {
	if !g.Fold {
		b.WriteString(`<tbody>`)
	} else {
		id := fmt.Sprintf("fold-%s-%d", r.foldPrefix, g.Index)
		b.WriteString(`<tbody class="ph-fold"><tr class="ph-fold-head"><td colspan="4">`)
		fmt.Fprintf(b, `<input type="checkbox" class="ph-fold-cb" id="%s"><label for="%s">%d unchanged lines</label>`,
			id, id, g.Hidden)
		b.WriteString(`</td></tr>`)
	}
	for _, row := range g.Rows {
		r.writeRow(b, row)
	}
	b.WriteString(`</tbody>`)
}

// writeRow renders one row of the table: two number cells, a sign, and the
// line.
func (r *docRenderer) writeRow(b *strings.Builder, row diffRow) {
	if row.Kind == rowNotes {
		r.writeNotesRow(b, row)
		return
	}

	b.WriteString(`<tr class="ph-row ph-r-`)
	b.WriteString(string(row.Kind))
	if row.Start {
		b.WriteString(" ph-blk-start")
	}
	if row.Block.Heading {
		b.WriteString(" ph-head")
	}
	if row.Region {
		b.WriteString(" ph-region")
	}
	if row.Folded {
		b.WriteString(" ph-folded")
	}
	// A marker row is the renderer talking, not the document: "paragraph moved
	// here (was line 3)" sits in the same column as the prose around it, and
	// without a class of its own it reads as a sentence someone wrote.
	if row.Note != "" {
		b.WriteString(" ph-marker")
	}
	b.WriteString(`"`)
	// The id goes on the block's first row and only there: an id repeated down a
	// block would give one anchor several places to scroll to, and an empty id=""
	// on an uncommentable row is a fragment that matches every such row at once.
	if row.Start && row.Block.Commentable {
		b.WriteString(` id="` + blockDOMID(row.Block.Anchor) + `"`)
	}
	writeBlockAttrs(b, row.Block)
	b.WriteString(`>`)

	writeNumCell(b, "ph-n-old", row.OldNum, row.OldEnd)
	writeNumCell(b, "ph-n-new", row.NewNum, row.NewEnd)
	b.WriteString(`<td class="ph-sign">` + signOf(row.Kind) + `</td>`)

	b.WriteString(`<td class="ph-text`)
	if row.Block.Mono {
		b.WriteString(" ph-mono")
	}
	b.WriteString(`">`)
	if row.Note != "" {
		// Renderer-authored text, but escaped all the same: a block's label
		// carries a code fence's info string, which the document wrote.
		b.WriteString(template.HTMLEscapeString(row.Note))
	} else {
		writeInlineSpans(b, row.Spans)
	}
	b.WriteString(`</td></tr>`)
}

// writeNotesRow renders a block's threads and, for the owner, the form that
// opens a new one, in a full-width row under the block's lines.
//
// It renders through html/template rather than by hand because everything here
// is prose someone else wrote; contextual auto-escaping is what keeps a comment
// body text. The template is executed into a buffer first, for the reason
// Server.render uses one: a template that fails halfway must not leave its
// half-written markup inside the diff.
//
// This is also where a block claims its threads. The row model only emits a
// notes row for a block that has something to put in it, so there is no case
// here for an empty one.
func (r *docRenderer) writeNotesRow(b *strings.Builder, row diffRow) {
	blk := row.Block
	threads := r.pending[blk.Key]
	delete(r.pending, blk.Key)

	// The anchor note states what a comment written here will attach to. The
	// selection a reviewer makes is by line and the anchor stored is by block, so
	// without this the indirection would be invisible — and it is server-rendered
	// rather than filled in by script, because it has to be readable before the
	// reviewer decides to type.
	path, index := anchorPathLabel(blk.Anchor.HeadingPath), blk.Anchor.Index

	data := blockComments{}
	for _, t := range threads {
		p := threadPanelOf(t, r.in.Controls)
		p.AnchorPath, p.AnchorIndex = path, index
		data.Threads = append(data.Threads, p)
	}
	if r.in.Controls.Owner {
		data.Compose = &composeForm{
			ActionBase:  r.in.Controls.ActionBase,
			DocPath:     r.in.Path,
			Ordinal:     blk.Key.ordinal,
			Side:        blk.Key.side,
			Hash:        blk.Anchor.BlockHash,
			AnchorPath:  path,
			AnchorIndex: index,
		}
	}

	var buf bytes.Buffer
	if err := blockThreadsTmpl.Execute(&buf, data); err != nil {
		slog.Error("rendering the comments on a diff block failed",
			"doc", r.in.Path, scribe.Err(err))
		return
	}
	b.WriteString(`<tr class="ph-notes`)
	// A notes row hides with the block it belongs to. Dropping the flag the row
	// model already set left a collapsed run displaying the compose forms of the
	// very blocks it had just hidden.
	if row.Folded {
		b.WriteString(" ph-folded")
	}
	b.WriteString(`"`)
	writeBlockAttrs(b, blk)
	b.WriteString(`><td colspan="4">`)
	b.Write(buf.Bytes())
	b.WriteString(`</td></tr>`)
}

// writeBlockAttrs writes the one data attribute a row of a commentable block
// carries: which block it belongs to.
//
// It used to write the block's heading path and index alongside, so that a
// script could read a row's section without walking back up the table. No
// script reads them, and an attribute pair repeated on every row of every diff
// for a reader that does not exist is the speculative chrome this port set out
// to remove. The heading path a person sees is in the composer and the thread
// header, where it is read.
//
// A row with no anchor gets no attribute at all rather than an empty one. The
// selection script clamps a drag to rows sharing a data-anchor, and rows that
// all carried data-anchor="" would look to it like one enormous block.
func writeBlockAttrs(b *strings.Builder, blk *rowBlock) {
	if !blk.Commentable {
		return
	}
	b.WriteString(` data-anchor="` + blockDOMID(blk.Anchor) + `"`)
}

// writeNumCell writes one of the two gutter tracks.
//
// A zero number is an empty cell: the row has no line number on this side, and
// the design's rule is that a number the renderer had to guess is never shown.
// A region row states the range it stands for instead, in the cell and in a
// title so it is readable when the track is too narrow for it.
func writeNumCell(b *strings.Builder, side string, n, end int) {
	b.WriteString(`<td class="ph-n ` + side + `"`)
	switch {
	case n == 0:
		b.WriteString(`></td>`)
	case end > n:
		fmt.Fprintf(b, ` title="lines %d–%d">%d–%d</td>`, n, end, n, end)
	default:
		fmt.Fprintf(b, `>%d</td>`, n)
	}
}

// signOf is the one character the sign column shows. It is where the add/delete
// tint starts, so the gutter never reads as part of the change.
func signOf(kind rowKind) string {
	switch kind {
	case rowInsert:
		return "+"
	case rowDelete:
		return "-"
	case rowMove:
		return "≡"
	}
	return ""
}

// blockAnchors numbers a revision's blocks the way the anchor model does:
// within their own heading path, not document-globally. core.AnchorBlocks is
// the single spelling of that numbering — service.AnchorOf reproduces it for
// the anchor a comment stores — so the id a block carries here and the anchor
// the form posts back cannot drift apart.
func blockAnchors(blocks []prosediff.Block) []core.AnchorBlock {
	hashes := make([]string, len(blocks))
	paths := make([][]string, len(blocks))
	for i, b := range blocks {
		hashes[i], paths[i] = b.Hash, b.HeadingPath
	}
	return core.AnchorBlocks(hashes, paths)
}

// blockDOMID is the id attribute a rendered block carries: a digest of its
// anchor tuple.
//
// Not its position on the page. An ordinal id renumbers whenever anything above
// it is inserted, so a link saved from one revision would silently scroll to a
// different paragraph in the next — exactly the relocation the anchor model
// refuses to do for comments, and the reader could not tell it had happened.
// The digest covers the whole tuple, block hash included, so a link into a block
// that has since been rewritten resolves to nothing at all rather than to its
// neighbour. The index keeps two blocks that repeat the same text under the same
// headings — "TBD" is written a dozen times in a real corpus — from sharing an
// id.
func blockDOMID(a core.CommentAnchor) string {
	h := sha256.New()
	// NUL separates the parts because it cannot occur in a heading, a path or a
	// hash, so no two distinct tuples can hash the same byte string.
	write := func(s string) {
		h.Write([]byte(s))
		h.Write([]byte{0})
	}
	write(a.DocID)
	write(string(a.Side))
	for _, seg := range a.HeadingPath {
		write(seg)
	}
	write(fmt.Sprint(a.Index))
	write(a.BlockHash)
	return "b-" + hex.EncodeToString(h.Sum(nil))[:16]
}

// sideOf defaults an empty side to the new one, the way service.CommentOn does
// when it stores a comment: an anchor read back without a side is a comment on
// the text under review.
func sideOf(s core.CommentSide) core.CommentSide {
	if s == core.SideOld {
		return core.SideOld
	}
	return core.SideNew
}

// writeInlineSpans renders a row's content, marking deletions and insertions
// where they sit. The Space flag reproduces prosediff's own spacing: a span
// that replaces the one before it carries Space=false, so a one-word
// substitution does not render with a gap in the middle.
func writeInlineSpans(b *strings.Builder, spans []prosediff.Span) {
	emitted := false
	for _, s := range spans {
		if s.Space && emitted {
			b.WriteByte(' ')
		}
		emitted = true
		writeSpan(b, s)
	}
}

// writeSpan writes one span's escaped text, wrapped in <del> or <ins> for a
// change and bare for an equal run.
func writeSpan(b *strings.Builder, s prosediff.Span) {
	esc := template.HTMLEscapeString(s.Text)
	switch s.Op {
	case prosediff.OpDelete:
		b.WriteString("<del>")
		b.WriteString(esc)
		b.WriteString("</del>")
	case prosediff.OpInsert:
		b.WriteString("<ins>")
		b.WriteString(esc)
		b.WriteString("</ins>")
	default:
		b.WriteString(esc)
	}
}