add encoding/hex and use it to expand the percent decoding chars

core/encoding/hex/hex.odin

@@ -0,0 +1,74 @@
+package hex
+
+import "core:strings"
+
+encode :: proc(src: []byte, allocator := context.allocator) -> []byte #no_bounds_check {
+	dst := make([]byte, len(src) * 2, allocator)
+    for i := 0; i < len(src); i += 1 {
+		v := src[i]
+        dst[i]   = HEXTABLE[v>>4]
+        dst[i+1] = HEXTABLE[v&0x0f]
+        i += 2
+    }
+
+	return dst
+}
+
+
+decode :: proc(src: []byte, allocator := context.allocator) -> (dst: []byte, ok: bool) #no_bounds_check {
+	if len(src) % 2 == 1 {
+		return
+	}
+
+	dst = make([]byte, len(src) / 2, allocator)
+	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
+
+		dst[i] = (a << 4) | b
+		i += 1
+	}
+
+	return dst, true
+}
+
+// Decodes the given sequence into one byte.
+// Should be called with one rune worth of the source, eg: 0x23 -> '#'.
+decode_sequence :: proc(str: string) -> (byte, bool) {
+	no_prefix_str := strings.trim_prefix(str, "0x")
+	val: byte
+	for i := 0; i < len(no_prefix_str); i += 1 {
+		index := (len(no_prefix_str) - 1) - i // reverse the loop.
+
+		hd, ok := hex_digit(no_prefix_str[i])
+		if !ok {
+			return 0, false
+		}
+
+		val += u8(hd) << uint(4 * index)
+	}
+
+	return val, true
+}
+
+@(private)
+HEXTABLE := [16]byte {
+    '0', '1', '2', '3',
+    '4', '5', '6', '7',
+    '8', '9', 'a', 'b',
+    'c', 'd', 'e', 'f',
+}
+
+@(private)
+hex_digit :: proc(char: byte) -> (u8, bool) {
+    switch char {
+    case '0' ..= '9': return char - '0', true
+    case 'a' ..= 'f': return char - 'a' + 10, true
+    case 'A' ..= 'F': return char - 'A' + 10, true
+    case:             return 0, false
+    }
+}
+