From 4e0a71c24c8a087ba71150b1cdb4a3c4e5a4e5d1 Mon Sep 17 00:00:00 2001
From: mo <mocompute@outlook.com>
Date: Sun, 16 Aug 2026 19:48:19 +1200
Subject: [PATCH] Add non-allocating hex.decode_into_buffer

Also minor fixes to documentation of hex.decode().

Adds tests of the new procedure with buffers the correct size, larger,
and too small.
---
 core/encoding/hex/hex.odin                 | 38 ++++++++++++++-
 tests/core/encoding/hex/test_core_hex.odin | 56 +++++++++++++++++++++-
 2 files changed, 91 insertions(+), 3 deletions(-)

diff --git a/core/encoding/hex/hex.odin b/core/encoding/hex/hex.odin
index c4726d9e9..f7a39a081 100644
--- a/core/encoding/hex/hex.odin
+++ b/core/encoding/hex/hex.odin
@@ -95,12 +95,12 @@ Decodes a hex sequence into a byte slice
 *Allocates Using Provided Allocator*
 
 Inputs:
-- dst: The hex sequence decoded into bytes
 - src: The `[]byte` to be hex-decoded
 - allocator: (default: context.allocator)
 - loc: The caller location for debugging purposes (default: #caller_location)
 
 Returns:
+- dst: The hex sequence decoded into bytes
 - ok:  A bool, `true` if decoding succeeded, `false` otherwise
 */
 decode :: proc(src: []byte, allocator := context.allocator, loc := #caller_location) -> (dst: []byte, ok: bool) {
@@ -123,6 +123,40 @@ decode :: proc(src: []byte, allocator := context.allocator, loc := #caller_locat
 	return dst, true
 }
 
+/*
+Decodes a hex sequence into a byte slice
+
+Inputs:
+- src: The `[]byte` to be hex-decoded
+- buf: A buffer large enough to hold the decoded sequence
+
+Returns:
+- dst: The hex sequence decoded into bytes
+- ok:  A bool, `true` if decoding succeeded, `false` otherwise
+*/
+decode_into_buffer :: proc(src: []byte, buf: []byte) -> (dst: []byte, ok: bool) #optional_ok {
+	if len(src) % 2 == 1 {
+		return
+	}
+	dst_len := len(src) / 2
+	if len(buf) < dst_len {
+		return
+	}
+
+	#no_bounds_check for i, j := 0, 1; j < len(src); j += 2 {
+		p := src[j-1]
+		q := src[j]
+
+		a := hex_digit(p) or_return
+		b := hex_digit(q) or_return
+
+		buf[i] = (a << 4) | b
+		i += 1
+	}
+
+	return buf[:dst_len], true
+}
+
 /*
 Decodes the first byte in a hex sequence to a byte
 
@@ -173,4 +207,4 @@ hex_digit :: proc(char: byte) -> (u8, bool) {
 	case 'A' ..= 'F': return char - 'A' + 10, true
 	case:             return 0, false
 	}
-}
\ No newline at end of file
+}
diff --git a/tests/core/encoding/hex/test_core_hex.odin b/tests/core/encoding/hex/test_core_hex.odin
index 6a00c9705..fdb3a2734 100644
--- a/tests/core/encoding/hex/test_core_hex.odin
+++ b/tests/core/encoding/hex/test_core_hex.odin
@@ -47,6 +47,60 @@ hex_decode :: proc(t: ^testing.T) {
 	}
 }
 
+@(test)
+hex_decode_into_buffer :: proc(t: ^testing.T) {
+	for test in CASES {
+		buffer := make([]u8, len(test[1]) / 2)
+		defer delete(buffer)
+		decoded, ok := hex.decode_into_buffer(transmute([]byte)test[1], buffer)
+		testing.expect(t, ok, "decode_into_buffer: not ok")
+		testing.expectf(
+			t,
+			ok,
+			"decode: %q not ok",
+			test[1],
+		)
+		testing.expectf(
+			t,
+			string(decoded) == test[0],
+			"decode: %q -> %q (should be: %q)",
+			test[1],
+			string(decoded),
+			test[0],
+		)
+	}
+
+	// destination buffer is larger
+	for test in CASES {
+		buffer := make([]u8, len(test[1]) / 2 + 1)
+		defer delete(buffer)
+		decoded, ok := hex.decode_into_buffer(transmute([]byte)test[1], buffer)
+		testing.expect(t, ok, "decode_into_buffer: not ok")
+		testing.expectf(
+			t,
+			ok,
+			"decode: %q not ok",
+			test[1],
+		)
+		testing.expectf(
+			t,
+			string(decoded) == test[0],
+			"decode: %q -> %q (should be: %q)",
+			test[1],
+			string(decoded),
+			test[0],
+		)
+	}
+
+	// destination buffer is too small
+	for test in CASES {
+		buffer := make([]u8, len(test[1]) / 2 - 1)
+		defer delete(buffer)
+		_, ok := hex.decode_into_buffer(transmute([]byte)test[1], buffer)
+		testing.expect(t, !ok, "decode_into_buffer: should not be ok")
+	}
+}
+
 @(test)
 hex_decode_sequence :: proc(t: ^testing.T) {
 	b, ok := hex.decode_sequence("0x23")
@@ -83,4 +137,4 @@ hex_decode_sequence :: proc(t: ^testing.T) {
 
 	_, ok = hex.decode_sequence("123")
 	testing.expect(t, !ok, "decode_sequence: 123 should be too long")
-}
\ No newline at end of file
+}