From 49566de7fbea2356b1b11075e76ffe9289ed1261 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Petr=20Balv=C3=ADn?= <opensource@petrbalvin.org>
Date: Wed, 5 Aug 2026 18:52:00 +0200
Subject: [PATCH] feat(asm): add did-you-mean suggestions for undefined labels

Assisted-by: DeepSeek V4 Pro
---
 asm/riscv_assemble.go | 72 ++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 68 insertions(+), 4 deletions(-)

diff --git a/asm/riscv_assemble.go b/asm/riscv_assemble.go
index a14f4e7..6999168 100644
--- a/asm/riscv_assemble.go
+++ b/asm/riscv_assemble.go
@@ -162,7 +162,7 @@ func encodeRISCVInstr(instr *ast.Instr, pc int, offsets map[string]int, fi riscv
 			target := labelFromOperand(ops[0])
 			targetOff, ok := offsets[target]
 			if !ok {
-				return nil, fmt.Errorf("undefined label %q", target)
+				return nil, fmt.Errorf("undefined label %q%s", target, suggestLabel(target, offsets))
 			}
 			offset := int32(targetOff - pc)
 			// AUIPC X1, upper 20 bits
@@ -187,7 +187,7 @@ func encodeRISCVInstr(instr *ast.Instr, pc int, offsets map[string]int, fi riscv
 		}
 		targetOff, ok := offsets[target]
 		if !ok {
-			return nil, fmt.Errorf("undefined label %q", target)
+			return nil, fmt.Errorf("undefined label %q%s", target, suggestLabel(target, offsets))
 		}
 		offset := int32(targetOff - pc)
 		// C.J: funct3=0x5, offset in ±2 KB, bit 0 must be 0.
@@ -208,7 +208,7 @@ func encodeRISCVInstr(instr *ast.Instr, pc int, offsets map[string]int, fi riscv
 		}
 		targetOff, ok := offsets[target]
 		if !ok {
-			return nil, fmt.Errorf("undefined label %q", target)
+			return nil, fmt.Errorf("undefined label %q%s", target, suggestLabel(target, offsets))
 		}
 		offset := int32(targetOff - pc)
 		// JAL X0, target → C.J when offset fits.
@@ -435,7 +435,7 @@ func encodeRISCVInstr(instr *ast.Instr, pc int, offsets map[string]int, fi riscv
 		target := labelFromOperand(ops[2])
 		targetOff, ok := offsets[target]
 		if !ok {
-			return nil, fmt.Errorf("undefined label %q", target)
+			return nil, fmt.Errorf("undefined label %q%s", target, suggestLabel(target, offsets))
 		}
 		offset := int32(targetOff - pc)
 		if rs1 < 0 || rs2 < 0 {
@@ -1064,3 +1064,67 @@ func labelFromOperand(op *ast.Operand) string {
 	}
 	return op.Raw
 }
+
+// suggestLabel returns a "did you mean" suggestion for an undefined label.
+func suggestLabel(target string, offsets map[string]int) string {
+	if len(offsets) == 0 {
+		return ""
+	}
+	// Find the closest matching label using Levenshtein distance.
+	bestDist := len(target) + 1
+	var best string
+	for name := range offsets {
+		dist := levenshtein(target, name)
+		if dist < bestDist {
+			bestDist = dist
+			best = name
+		}
+	}
+	// Only suggest if the distance is small enough.
+	if bestDist <= 3 && bestDist < len(target)/2+1 {
+		return fmt.Sprintf(" — did you mean %q?", best)
+	}
+	return ""
+}
+
+// levenshtein computes the Levenshtein distance between two strings.
+func levenshtein(a, b string) int {
+	la, lb := len(a), len(b)
+	if la == 0 {
+		return lb
+	}
+	if lb == 0 {
+		return la
+	}
+	// Create a matrix of distances.
+	prev := make([]int, lb+1)
+	curr := make([]int, lb+1)
+	for j := 0; j <= lb; j++ {
+		prev[j] = j
+	}
+	for i := 1; i <= la; i++ {
+		curr[0] = i
+		for j := 1; j <= lb; j++ {
+			cost := 1
+			if a[i-1] == b[j-1] {
+				cost = 0
+			}
+			curr[j] = min3(curr[j-1]+1, prev[j]+1, prev[j-1]+cost)
+		}
+		prev, curr = curr, prev
+	}
+	return prev[lb]
+}
+
+func min3(a, b, c int) int {
+	if a < b {
+		if a < c {
+			return a
+		}
+		return c
+	}
+	if b < c {
+		return b
+	}
+	return c
+}