Fix the string scans on big-endian targets with Zig 0.16

.github/workflows/build.yml

@@ -21,3 +21,10 @@ jobs:
       - run: zig fmt --check .
       - run: zig build test --summary all
       - run: zig build conformance
+      # The string scans depend on how a bool vector maps to an integer,
+      # which differs on big-endian targets.
+      - name: Test on big-endian (s390x)
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y qemu-user-static
+          zig build test -Dtarget=s390x-linux -fqemu --summary all

CHANGELOG.md

@@ -10,6 +10,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 - Support for Zig 0.17, alongside 0.16
 
+### Fixed
+- Strings were scanned wrong on big-endian targets with Zig 0.16, so decoding failed and encoding could leave characters unescaped
+
 ## [0.1.0] - 2026-09-16
 
 Initial release.

src/compat.zig

@@ -1,8 +1,8 @@
-//! Type reflection helpers that work on both Zig 0.16 and 0.17.
+//! Helpers that work on both Zig 0.16 and 0.17.
 //!
 //! Zig 0.17 changed `@typeInfo` for containers from a slice of field structs
 //! (`fields`) to parallel slices (`field_names`, `field_types`,
-//! `field_attrs`). These helpers present the 0.16 shape on both.
+//! `field_attrs`). The reflection helpers present the 0.16 shape on both.
 
 const std = @import("std");
 
@@ -58,3 +58,14 @@ pub fn unionFields(comptime T: type) []const UnionField {
         break :blk &final;
     };
 }
+
+/// Bit-casts a bool vector to an integer with element `i` in bit `i`, so that
+/// `@ctz` finds the first true element. Zig 0.16 bit-casts element 0 to the
+/// most significant bit on big-endian targets, while 0.17 always puts it in
+/// the least significant one.
+pub inline fn vectorMask(comptime n: usize, v: @Vector(n, bool)) @Int(.unsigned, n) {
+    const Mask = @Int(.unsigned, n);
+    const reversed = comptime @as(Mask, @bitCast(std.simd.iota(u8, n) == @as(@Vector(n, u8), @splat(0)))) != 1;
+    const mask: Mask = @bitCast(v);
+    return if (reversed) @bitReverse(mask) else mask;
+}

src/decode.zig

@@ -641,7 +641,7 @@ pub const Decoder = struct {
         while (i + 16 <= window.len) : (i += 16) {
             const chunk: V = window[i..][0..16].*;
             acc |= chunk;
-            const hits: u16 = @bitCast((chunk == quote) | (chunk == backslash) | (chunk < space));
+            const hits = compat.vectorMask(16, (chunk == quote) | (chunk == backslash) | (chunk < space));
             if (hits != 0) return .{ .end = i + @ctz(hits), .non_ascii = @reduce(.Or, acc) >= 0x80 };
         }
         var tail: u8 = @reduce(.Or, acc);

src/encode.zig

@@ -322,7 +322,7 @@ pub fn writeString(str: []const u8, w: *Writer) Writer.Error!void {
     var i: usize = 0;
     while (i + 16 <= str.len) {
         const chunk: V = str[i..][0..16].*;
-        const hits: u16 = @bitCast((chunk == quote) | (chunk == backslash) | (chunk < space));
+        const hits = compat.vectorMask(16, (chunk == quote) | (chunk == backslash) | (chunk < space));
         if (hits == 0) {
             i += 16;
             continue;