// ingestion/internal/wiki/merge.go
package wiki

import (
	"fmt"
	"sort"
	"strings"
)

var bulletSections = map[string]bool{
	"Related Concepts":                  true,
	"Related Entities":                  true,
	"Sources":                           true,
	"Key Claims":                        true,
	"Entities Mentioned":                true,
	"Concepts Introduced or Reinforced": true,
	"Chapters":                          true,
}

var appendSections = map[string]bool{
	"Evolving Notes":        true,
	"Updates":               true,
	"Open Questions Raised": true,
	"Open Questions":        true,
}

type section struct {
	heading string
	content string
}

// Merge combines two Page values with the same path.
// Frontmatter is taken from a. The preamble (content before the first ##
// heading) is taken from a — same "on-disk wins" default every unlisted
// named section gets, kept as a separate field rather than a section with
// an empty heading so it can never collide with one (see parseSections).
// Sections are merged by strategy: bullet sections union unique lines,
// append sections concatenate, all others keep a's version. Sections in b
// not present in a are appended.
// Merge discards any disagreement silently; prefer MergeReport where the
// caller can act on it.
func Merge(a, b Page) Page {
	page, _ := MergeReport(a, b)
	return page
}

// MergeReport is Merge, plus the headings of sections where b carried a
// different version of a section a already had — and lost.
//
// That loss is the property which makes ingest safe to run unattended (#103):
// a regenerating writer can add to a note but never rewrite it. But the
// discarded text is also a signal, and until now it went in the bin: the source
// now says something different about a section we already hold. Nobody could
// tell "the source is unchanged" from "the source changed its mind and we
// ignored it".
//
// Deliberately NOT a semantic contradiction check. This reports textual
// disagreement, which is cheap, deterministic, and needs no model on the write
// path. Deciding whether a disagreement is a genuine contradiction stays with
// the Judge-gated enrich pass that already exists (brain-gardener#175), which
// is approval-gated by design — detection here, adjudication there.
func MergeReport(a, b Page) (Page, []string) {
	fmA, preambleA, secsA := parseSections(a.Content)
	fmB, _, secsB := parseSections(b.Content)

	var conflicts []string
	idx := make(map[string]int, len(secsA))
	for i, s := range secsA {
		idx[s.heading] = i
	}

	for _, sB := range secsB {
		i, exists := idx[sB.heading]
		if !exists {
			idx[sB.heading] = len(secsA)
			secsA = append(secsA, sB)
			continue
		}
		sA := secsA[i]
		switch {
		case bulletSections[sB.heading]:
			secsA[i].content = mergeBullets(sA.content, sB.content)
		case appendSections[sB.heading]:
			secsA[i].content = appendNew(sA.content, sB.content)
		default:
			// a wins — but say that b disagreed, rather than dropping it mute.
			if strings.TrimSpace(sA.content) != strings.TrimSpace(sB.content) {
				conflicts = append(conflicts, sB.heading)
			}
		}
	}

	return Page{Path: a.Path, Content: rebuildContent(refreshOwned(fmA, fmB), preambleA, secsA)}, conflicts
}

// generatorOwned names the frontmatter fields the ingest pipeline may refresh
// on a page it is revising. Everything else on disk wins.
//
// #103 made the on-disk frontmatter win wholesale, which is what stops a
// regenerating writer from overwriting human-set metadata — a title someone
// fixed, or the claim-property envelope (brain#21). The cost was that fields
// the generator legitimately owns froze at their first-ingest values: when it
// last ran, and what it last saw.
//
// An explicit allowlist rather than a carve-out: the ownership boundary is
// nameable, and a field nobody has thought about stays human-owned by default,
// which is the safe direction.
var generatorOwned = map[string]bool{
	"last_updated": true,
	"source_hash":  true,
}

// fmLine is one line of a frontmatter block: a parsed key/value pair, or a
// raw line (blank, comment, or anything else that isn't a simple "key:
// value") kept verbatim. Same shape as api.frontmatter's fmLine — this
// package doesn't import that one (it's unexported there, and a's/b's
// frontmatter here always arrives as a string, not a parsed struct), so the
// small, already-proven set()-appends-if-absent behavior is reproduced
// locally rather than string-index surgery reinvented per call site.
type fmLine struct {
	key, value, raw string
}

// parseFrontmatterBlock parses a "---\n...\n---\n" fenced block (the shape
// parseSections produces) into an ordered line list. A block with no
// opening/closing fence — including the empty string, a page with no
// frontmatter at all — returns nil, ok=false: there is nothing to update in
// place, only something to potentially create.
func parseFrontmatterBlock(block string) (lines []fmLine, ok bool) {
	if !strings.HasPrefix(block, "---\n") {
		return nil, false
	}
	rest := block[len("---\n"):]
	end := strings.Index(rest, "\n---\n")
	if end < 0 {
		return nil, false
	}
	for _, line := range strings.Split(rest[:end], "\n") {
		// An indented line is a continuation (a YAML block-list item, a
		// wrapped value) never a top-level key, regardless of whether it
		// happens to contain a colon — "  - 'Chapter 3: The Reckoning'"
		// is not a key named "- 'Chapter 3". Checked before Cut so a
		// colon inside one never reaches the key/value path at all.
		if line != "" && (line[0] == ' ' || line[0] == '\t') {
			lines = append(lines, fmLine{raw: line})
			continue
		}
		key, val, found := strings.Cut(line, ":")
		key = strings.TrimSpace(key)
		if !found || key == "" || strings.HasPrefix(strings.TrimSpace(line), "#") {
			lines = append(lines, fmLine{raw: line})
			continue
		}
		lines = append(lines, fmLine{key: key, value: strings.TrimSpace(val)})
	}
	return lines, true
}

// setLine updates key's value in place if lines already has it, or appends
// a new entry if it doesn't — the one behavior the original refreshOwned
// got wrong (PR #126 review, round 2: it only ever updated in place).
func setLine(lines []fmLine, key, value string) []fmLine {
	for i := range lines {
		if lines[i].key == key {
			lines[i].value = value
			return lines
		}
	}
	return append(lines, fmLine{key: key, value: value})
}

func renderFrontmatterBlock(lines []fmLine) string {
	if len(lines) == 0 {
		return ""
	}
	var sb strings.Builder
	sb.WriteString("---\n")
	for _, l := range lines {
		switch {
		case l.key == "":
			sb.WriteString(l.raw)
		case l.value == "":
			// A key with nothing after the colon (the YAML block-list shape:
			// "aliases:" followed by indented "- item" lines on their own
			// raw lines) must stay bare — "key: " with a trailing space
			// is a different, if invisible, byte sequence, and this file's
			// whole purpose is byte-stability across an unrelated merge.
			sb.WriteString(l.key)
			sb.WriteByte(':')
		default:
			sb.WriteString(l.key)
			sb.WriteString(": ")
			sb.WriteString(l.value)
		}
		sb.WriteByte('\n')
	}
	sb.WriteString("---\n")
	return sb.String()
}

// refreshOwned returns a's frontmatter with the generator-owned fields taken
// from b where b supplies them. Fields absent from b keep a's value, so a
// generator that stops emitting a field cannot blank it.
//
// A field b supplies that a doesn't have yet — every page ingested before
// this mechanism existed has no source_hash line at all — is appended, not
// dropped (round 1 of review: dropping it silently meant source_hash never
// populated for the pre-existing corpus). If a has no frontmatter fence at
// all, one is created to hold the new fields rather than splicing raw
// "key: value" text into the body (round 2: that corrupted the page and the
// fields would duplicate on every subsequent merge, since unfenced text
// isn't recognized as frontmatter by parseSections on the next pass).
func refreshOwned(a, b string) string {
	bLines, _ := parseFrontmatterBlock(b)
	bVals := make(map[string]string)
	for _, l := range bLines {
		if l.key != "" && generatorOwned[l.key] {
			bVals[l.key] = l.value
		}
	}
	if len(bVals) == 0 {
		return a
	}

	aLines, _ := parseFrontmatterBlock(a) // ok==false (no fence) -> aLines nil, same as "start fresh"

	// Sorted for a deterministic, byte-stable result when more than one
	// field is appended in the same call — map iteration order isn't.
	keys := make([]string, 0, len(bVals))
	for k := range bVals {
		keys = append(keys, k)
	}
	sort.Strings(keys)
	for _, k := range keys {
		aLines = setLine(aLines, k, bVals[k])
	}
	return renderFrontmatterBlock(aLines)
}

// parseSections splits markdown into its frontmatter, its preamble (content
// before the first `## ` heading), and its `## `-headed sections.
//
// The preamble is returned separately rather than as a section with an
// empty heading — a section keyed by heading=="" would collide with a
// literal degenerate `## ` (marker, no title text; strings.TrimPrefix also
// yields ""), which is a distinct, addressable section a page can genuinely
// have, not the same thing as "no heading yet." Two entries sharing one key
// in Merge's idx map means one of them silently loses lookup visibility —
// found in review as real, reproducible data loss, not a hypothetical.
func parseSections(markdown string) (frontmatter, preamble string, sections []section) {
	lines := strings.Split(markdown, "\n")
	i := 0

	if i < len(lines) && strings.TrimSpace(lines[i]) == "---" {
		i++
		var fmLines []string
		for i < len(lines) {
			if strings.TrimSpace(lines[i]) == "---" {
				i++
				break
			}
			fmLines = append(fmLines, lines[i])
			i++
		}
		frontmatter = fmt.Sprintf("---\n%s\n---\n", strings.Join(fmLines, "\n"))
	}

	// Everything before the first `## ` heading — an intro paragraph, or, if
	// the page has no headings at all, the entire body — is ordinary content,
	// not something this parser is entitled to drop (hyperguild#103 review:
	// dropping it meant Merge, one caller up, would silently replace the
	// whole on-disk body with the fresh extraction whenever a page didn't
	// follow the ## convention — exactly the class of bug this file exists
	// to prevent).
	var preambleLines []string
	for ; i < len(lines); i++ {
		if strings.HasPrefix(lines[i], "## ") {
			break
		}
		preambleLines = append(preambleLines, lines[i])
	}
	preamble = strings.Join(preambleLines, "\n")

	var cur *section
	for ; i < len(lines); i++ {
		line := lines[i]
		if strings.HasPrefix(line, "## ") {
			if cur != nil {
				sections = append(sections, *cur)
			}
			cur = &section{heading: strings.TrimPrefix(line, "## ")}
		} else if cur != nil {
			cur.content += line + "\n"
		}
	}
	if cur != nil {
		sections = append(sections, *cur)
	}
	return
}

// rebuildContent serialises sections back into a page.
//
// Each section body is normalised to exactly one trailing newline and no
// leading blank line. Without that, a section keeps whatever blank lines it was
// parsed with and this function adds its own, so every merge pass grows the page
// by a blank line — invisible in a diff viewer, fatal to byte-stability. Compile-on-ingest
// merges on every run, so "merging changes nothing when nothing changed" has to
// hold at the byte level, not just the content level.
func rebuildContent(frontmatter, preamble string, sections []section) string {
	var sb strings.Builder
	sb.WriteString(frontmatter)
	if body := strings.Trim(preamble, "\n"); body != "" {
		fmt.Fprintf(&sb, "\n%s\n", body)
	}
	for _, sec := range sections {
		body := strings.Trim(sec.content, "\n")
		fmt.Fprintf(&sb, "\n## %s\n\n%s\n", sec.heading, body)
	}
	return sb.String()
}

// canonicalLine collapses a line to its comparison key: internal whitespace
// runs (a re-extraction's bullet spacing, tabs vs spaces, trailing spaces)
// must not make the same logical line look new. strings.Fields already
// trims and splits on any whitespace run, so joining it back with single
// spaces is exactly "same words, same order, whitespace-insensitive."
func canonicalLine(line string) string {
	return strings.Join(strings.Fields(line), " ")
}

// appendNew concatenates the lines of b that a does not already carry.
//
// Append sections used to concatenate wholesale, which is correct exactly once.
// Compile-on-ingest merges a freshly extracted page into the one already on
// disk, so an unchanged source re-ingested would grow "Open Questions" and
// "Updates" on every run until the page is unreadable — and the drift guard
// that would otherwise catch it (hashing the source body) does not exist yet,
// so idempotency has to hold here rather than upstream.
//
// Line-level rather than block-level: the common case is a re-ingest carrying
// one genuinely new line alongside several it already has. A block-level
// "skip if identical" would duplicate the old lines every time that happened.
//
// The cost is that a line repeated verbatim on purpose collapses to one. For
// these sections — questions, updates, evolving notes — a repeat is noise
// rather than signal, and the failure is conservative: the page stops growing
// instead of growing wrongly.
func appendNew(a, b string) string {
	seen := make(map[string]bool)
	for _, line := range strings.Split(a, "\n") {
		if c := canonicalLine(line); c != "" {
			seen[c] = true
		}
	}

	var add []string
	for _, line := range strings.Split(b, "\n") {
		c := canonicalLine(line)
		if c == "" || seen[c] {
			continue
		}
		seen[c] = true
		add = append(add, line)
	}
	if len(add) == 0 {
		return a // nothing new — leave the section byte-identical
	}
	return strings.TrimRight(a, "\n") + "\n\n" + strings.Join(add, "\n") + "\n"
}

func mergeBullets(a, b string) string {
	seen := make(map[string]bool)
	var lines []string
	for _, line := range strings.Split(a+b, "\n") {
		c := canonicalLine(line)
		if c == "" || seen[c] {
			continue
		}
		seen[c] = true
		lines = append(lines, line)
	}
	return strings.Join(lines, "\n") + "\n"
}