fix: align Huffman tree ordering across languages

Author: LessUp

changelog/2026-03-13_huffman-cross-language-fix.md

@@ -0,0 +1,13 @@
+# Huffman 跨语言互通修正
+
+日期：2026-03-13
+
+## 变更内容
+
+- 统一 C++、Go、Rust 三个 Huffman 实现的树构建平局决策
+- 在合并节点时保留子树最小符号作为排序键，避免不同语言生成不兼容码表
+- 修复 correctness workflow 中 `C++ encode -> Go/Rust decode` 的跨语言失败
+
+## 背景
+
+三个实现都使用“频率优先、符号次序兜底”的最小堆来构建 Huffman 树，但内部节点此前统一写死 `symbol = 0`，会让不同语言在频率相同的场景下以不同顺序继续合并，最终得到各自 round-trip 正常、跨语言互解失败的码表。此次将内部节点排序键收敛到一致规则后，跨语言输出恢复兼容。

huffman/cpp/main.cpp

@@ -111,7 +111,7 @@ static Node* build_tree(const std::vector<uint32_t>& freq) {
         Node* only = pq.top();
         pq.pop();
         Node* parent = new Node;
-        parent->symbol = 0;
+        parent->symbol = only->symbol;
         parent->freq = only->freq;
         parent->left = only;
         parent->right = nullptr;
@@ -123,7 +123,7 @@ static Node* build_tree(const std::vector<uint32_t>& freq) {
         Node* b = pq.top();
         pq.pop();
         Node* parent = new Node;
-        parent->symbol = 0;
+        parent->symbol = a->symbol < b->symbol ? a->symbol : b->symbol;
         parent->freq = a->freq + b->freq;
         parent->left = a;
         parent->right = b;

huffman/go/main.go

@@ -70,14 +70,18 @@ func buildTree(freq []uint32) *Node {
 	}
 	if h.Len() == 1 {
 		only := heap.Pop(h).(*Node)
-		parent := &Node{symbol: 0, freq: only.freq, left: only, right: nil}
+		parent := &Node{symbol: only.symbol, freq: only.freq, left: only, right: nil}
 		heap.Push(h, parent)
 	}
 	for h.Len() > 1 {
 		a := heap.Pop(h).(*Node)
 		b := heap.Pop(h).(*Node)
+		minSymbol := a.symbol
+		if b.symbol < minSymbol {
+			minSymbol = b.symbol
+		}
 		parent := &Node{
-			symbol: 0,
+			symbol: minSymbol,
 			freq:   a.freq + b.freq,
 			left:   a,
 			right:  b,

huffman/rust/main.rs

@@ -78,7 +78,7 @@ fn build_tree(freq: &[u32]) -> Box<Node> {
         let item = heap.pop().unwrap();
         let only = item.node;
         let parent = Box::new(Node {
-            symbol: 0,
+            symbol: only.symbol,
             freq: only.freq,
             left: Some(only),
             right: None,
@@ -94,7 +94,7 @@ fn build_tree(freq: &[u32]) -> Box<Node> {
         let b = heap.pop().unwrap().node;
         let freq_sum = a.freq + b.freq;
         let parent = Box::new(Node {
-            symbol: 0,
+            symbol: a.symbol.min(b.symbol),
             freq: freq_sum,
             left: Some(a),
             right: Some(b),