feat(reconcile): minimal word-level diff (Myers) with a bounded edit distance

Prefix/suffix trimmed first; adjacent deletes and inserts read as one Replace;
beyond 3,000 edits (two texts sharing almost nothing) the middle is one Replace
instead of gigabytes of trace. Mutation-checked: a wrong tie-break, missing
replace coalescing, wrong suffix accounting and a punctuation-bearing key each
turn tests red.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/reconcile/diff.go

@@ -0,0 +1,187 @@
+package reconcile
+
+// OpKind is the kind of one run in an edit script.
+type OpKind int
+
+const (
+	// Equal: A[AStart:AEnd] and B[BStart:BEnd] are the same words.
+	Equal OpKind = iota
+	// Replace: different words on both sides.
+	Replace
+	// Delete: words only in A.
+	Delete
+	// Insert: words only in B.
+	Insert
+)
+
+// Op is one run of the edit script. The ops of a Diff tile both inputs: each op starts
+// where the previous one ended, in A and in B.
+type Op struct {
+	Kind         OpKind
+	AStart, AEnd int
+	BStart, BEnd int
+}
+
+// maxEdits bounds the Myers search. Memory grows with the square of the edit
+// distance, so two texts that share almost nothing (one engine produced garbage)
+// are reported as one Replace instead of spending gigabytes proving it. A real
+// hour disagrees in roughly 400 places, i.e. about 800 edits, far below this.
+const maxEdits = 3000
+
+// Diff returns a minimal edit script from a to b (Myers, O(ND)). Adjacent deletions
+// and insertions are reported as one Replace.
+func Diff(a, b []string) []Op {
+	pre := 0
+	for pre < len(a) && pre < len(b) && a[pre] == b[pre] {
+		pre++
+	}
+	suf := 0
+	for suf < len(a)-pre && suf < len(b)-pre && a[len(a)-1-suf] == b[len(b)-1-suf] {
+		suf++
+	}
+	midA, midB := a[pre:len(a)-suf], b[pre:len(b)-suf]
+
+	steps, ok := myers(midA, midB)
+	if !ok {
+		steps = steps[:0]
+		for range midA {
+			steps = append(steps, stepDel)
+		}
+		for range midB {
+			steps = append(steps, stepIns)
+		}
+	}
+
+	var ops []Op
+	ai, bi := 0, 0
+	emit := func(k OpKind, da, db int) {
+		if da == 0 && db == 0 {
+			return
+		}
+		if k == Equal && len(ops) > 0 && ops[len(ops)-1].Kind == Equal {
+			ops[len(ops)-1].AEnd += da
+			ops[len(ops)-1].BEnd += db
+		} else {
+			ops = append(ops, Op{Kind: k, AStart: ai, AEnd: ai + da, BStart: bi, BEnd: bi + db})
+		}
+		ai += da
+		bi += db
+	}
+	emit(Equal, pre, pre)
+	for i := 0; i < len(steps); {
+		if steps[i] == stepEq {
+			j := i
+			for j < len(steps) && steps[j] == stepEq {
+				j++
+			}
+			emit(Equal, j-i, j-i)
+			i = j
+			continue
+		}
+		dels, inss := 0, 0
+		for i < len(steps) && steps[i] != stepEq {
+			if steps[i] == stepDel {
+				dels++
+			} else {
+				inss++
+			}
+			i++
+		}
+		switch {
+		case dels > 0 && inss > 0:
+			emit(Replace, dels, inss)
+		case dels > 0:
+			emit(Delete, dels, 0)
+		default:
+			emit(Insert, 0, inss)
+		}
+	}
+	emit(Equal, suf, suf)
+	return ops
+}
+
+type step int8
+
+const (
+	stepEq step = iota
+	stepDel
+	stepIns
+)
+
+// myers returns one step per element of the edit script, in order. ok is false when
+// the edit distance exceeds maxEdits.
+func myers(a, b []string) ([]step, bool) {
+	n, m := len(a), len(b)
+	if n == 0 && m == 0 {
+		return nil, true
+	}
+	limit := min(n+m, maxEdits)
+	off := limit + 1
+	v := make([]int32, 2*limit+3)
+	var trace [][]int32 // trace[d] is v as it stood at the start of round d, for k in [-d, d]
+
+	for d := 0; d <= limit; d++ {
+		snap := make([]int32, 2*d+1)
+		for k := -d; k <= d; k++ {
+			snap[k+d] = v[off+k]
+		}
+		trace = append(trace, snap)
+		for k := -d; k <= d; k += 2 {
+			var x int
+			if k == -d || (k != d && v[off+k-1] < v[off+k+1]) {
+				x = int(v[off+k+1])
+			} else {
+				x = int(v[off+k-1]) + 1
+			}
+			y := x - k
+			for x < n && y < m && a[x] == b[y] {
+				x++
+				y++
+			}
+			v[off+k] = int32(x)
+			if x >= n && y >= m {
+				return backtrack(trace, n, m, d), true
+			}
+		}
+	}
+	return nil, false
+}
+
+func backtrack(trace [][]int32, n, m, d int) []step {
+	var rev []step
+	x, y := n, m
+	for ; d >= 0; d-- {
+		snap := trace[d]
+		at := func(k int) int { return int(snap[k+d]) }
+		k := x - y
+		var prevK int
+		if k == -d || (k != d && at(k-1) < at(k+1)) {
+			prevK = k + 1
+		} else {
+			prevK = k - 1
+		}
+		prevX := 0
+		if d > 0 {
+			prevX = at(prevK)
+		}
+		prevY := prevX - prevK
+		for x > prevX && y > prevY {
+			rev = append(rev, stepEq)
+			x--
+			y--
+		}
+		if d > 0 {
+			if x == prevX {
+				rev = append(rev, stepIns)
+			} else {
+				rev = append(rev, stepDel)
+			}
+		}
+		x, y = prevX, prevY
+	}
+	out := make([]step, len(rev))
+	for i, s := range rev {
+		out[len(rev)-1-i] = s
+	}
+	return out
+}