std.net: Fix IPv6 address format compression for long zero runs (#22441)

Author: bondehagen

lib/std/net.zig

@@ -683,17 +683,50 @@ pub const Ip6Address = extern struct {
                 break :blk buf;
             },
         };
+
+        // Find the longest zero run
+        var longest_start: usize = 8;
+        var longest_len: usize = 0;
+        var current_start: usize = 0;
+        var current_len: usize = 0;
+
+        for (native_endian_parts, 0..) |part, i| {
+            if (part == 0) {
+                if (current_len == 0) {
+                    current_start = i;
+                }
+                current_len += 1;
+                if (current_len > longest_len) {
+                    longest_start = current_start;
+                    longest_len = current_len;
+                }
+            } else {
+                current_len = 0;
+            }
+        }
+
+        // Only compress if the longest zero run is 2 or more
+        if (longest_len < 2) {
+            longest_start = 8;
+            longest_len = 0;
+        }
+
         try out_stream.writeAll("[");
         var i: usize = 0;
         var abbrv = false;
         while (i < native_endian_parts.len) : (i += 1) {
-            if (native_endian_parts[i] == 0) {
+            if (i == longest_start) {
+                // Emit "::" for the longest zero run
                 if (!abbrv) {
                     try out_stream.writeAll(if (i == 0) "::" else ":");
                     abbrv = true;
                 }
+                i += longest_len - 1; // Skip the compressed range
                 continue;
             }
+            if (abbrv) {
+                abbrv = false;
+            }
             try std.fmt.format(out_stream, "{x}", .{native_endian_parts[i]});
             if (i != native_endian_parts.len - 1) {
                 try out_stream.writeAll(":");

lib/std/net/test.zig

@@ -26,6 +26,62 @@ test "parse and render IP addresses at comptime" {
     }
 }
 
+test "format IPv6 address with no zero runs" {
+    if (builtin.os.tag == .wasi) return error.SkipZigTest;
+
+    const addr = try std.net.Address.parseIp6("2001:db8:1:2:3:4:5:6", 0);
+
+    var buffer: [50]u8 = undefined;
+    const result = std.fmt.bufPrint(buffer[0..], "{}", .{addr}) catch unreachable;
+
+    try std.testing.expectEqualStrings("[2001:db8:1:2:3:4:5:6]:0", result);
+}
+
+test "parse IPv6 addresses and check compressed form" {
+    if (builtin.os.tag == .wasi) return error.SkipZigTest;
+
+    const alloc = testing.allocator;
+
+    // 1) Parse an IPv6 address that should compress to [2001:db8::1:0:0:2]:0
+    const addr1 = try std.net.Address.parseIp6("2001:0db8:0000:0000:0001:0000:0000:0002", 0);
+
+    // 2) Parse an IPv6 address that should compress to [2001:db8::1:2]:0
+    const addr2 = try std.net.Address.parseIp6("2001:0db8:0000:0000:0000:0000:0001:0002", 0);
+
+    // 3) Parse an IPv6 address that should compress to [2001:db8:1:0:1::2]:0
+    const addr3 = try std.net.Address.parseIp6("2001:0db8:0001:0000:0001:0000:0000:0002", 0);
+
+    // Print each address in Zig's default "[ipv6]:port" form.
+    const printed1 = try std.fmt.allocPrint(alloc, "{any}", .{addr1});
+    defer testing.allocator.free(printed1);
+    const printed2 = try std.fmt.allocPrint(alloc, "{any}", .{addr2});
+    defer testing.allocator.free(printed2);
+    const printed3 = try std.fmt.allocPrint(alloc, "{any}", .{addr3});
+    defer testing.allocator.free(printed3);
+
+    // Check the exact compressed forms we expect.
+    try std.testing.expectEqualStrings("[2001:db8::1:0:0:2]:0", printed1);
+    try std.testing.expectEqualStrings("[2001:db8::1:2]:0", printed2);
+    try std.testing.expectEqualStrings("[2001:db8:1:0:1::2]:0", printed3);
+}
+
+test "parse IPv6 address, check raw bytes" {
+    if (builtin.os.tag == .wasi) return error.SkipZigTest;
+
+    const expected_raw: [16]u8 = .{
+        0x20, 0x01, 0x0d, 0xb8, // 2001:db8
+        0x00, 0x00, 0x00, 0x00, // :0000:0000
+        0x00, 0x01, 0x00, 0x00, // :0001:0000
+        0x00, 0x00, 0x00, 0x02, // :0000:0002
+    };
+
+    const addr = try std.net.Address.parseIp6("2001:db8:0000:0000:0001:0000:0000:0002", 0);
+
+    const actual_raw = addr.in6.sa.addr[0..];
+    try std.testing.expectEqualSlices(u8, expected_raw[0..], actual_raw);
+
+}
+
 test "parse and render IPv6 addresses" {
     if (builtin.os.tag == .wasi) return error.SkipZigTest;