From 52f3c6bc0a3d192d9f6ee9b3bf8c0a096a0e8986 Mon Sep 17 00:00:00 2001
From: Rutger Broekhoff
Date: Sat, 22 May 2021 23:49:01 +0200
Subject: Generate CRC-16 table at compile time

---
 src/crc16.zig | 60 ++++++++++++++++++++++-------------------------------------
 1 file changed, 22 insertions(+), 38 deletions(-)

(limited to 'src/crc16.zig')

diff --git a/src/crc16.zig b/src/crc16.zig
index 3b66d99..b00c795 100644
--- a/src/crc16.zig
+++ b/src/crc16.zig
@@ -1,45 +1,31 @@
 const Error = error{InvalidChecksum};
 
-// TODO(rutgerbrf): generate this table at compile time?
-const crc16tab = [256]u16{
-    0x0000, 0x1021, 0x2042, 0x3063, 0x4084, 0x50a5, 0x60c6, 0x70e7,
-    0x8108, 0x9129, 0xa14a, 0xb16b, 0xc18c, 0xd1ad, 0xe1ce, 0xf1ef,
-    0x1231, 0x0210, 0x3273, 0x2252, 0x52b5, 0x4294, 0x72f7, 0x62d6,
-    0x9339, 0x8318, 0xb37b, 0xa35a, 0xd3bd, 0xc39c, 0xf3ff, 0xe3de,
-    0x2462, 0x3443, 0x0420, 0x1401, 0x64e6, 0x74c7, 0x44a4, 0x5485,
-    0xa56a, 0xb54b, 0x8528, 0x9509, 0xe5ee, 0xf5cf, 0xc5ac, 0xd58d,
-    0x3653, 0x2672, 0x1611, 0x0630, 0x76d7, 0x66f6, 0x5695, 0x46b4,
-    0xb75b, 0xa77a, 0x9719, 0x8738, 0xf7df, 0xe7fe, 0xd79d, 0xc7bc,
-    0x48c4, 0x58e5, 0x6886, 0x78a7, 0x0840, 0x1861, 0x2802, 0x3823,
-    0xc9cc, 0xd9ed, 0xe98e, 0xf9af, 0x8948, 0x9969, 0xa90a, 0xb92b,
-    0x5af5, 0x4ad4, 0x7ab7, 0x6a96, 0x1a71, 0x0a50, 0x3a33, 0x2a12,
-    0xdbfd, 0xcbdc, 0xfbbf, 0xeb9e, 0x9b79, 0x8b58, 0xbb3b, 0xab1a,
-    0x6ca6, 0x7c87, 0x4ce4, 0x5cc5, 0x2c22, 0x3c03, 0x0c60, 0x1c41,
-    0xedae, 0xfd8f, 0xcdec, 0xddcd, 0xad2a, 0xbd0b, 0x8d68, 0x9d49,
-    0x7e97, 0x6eb6, 0x5ed5, 0x4ef4, 0x3e13, 0x2e32, 0x1e51, 0x0e70,
-    0xff9f, 0xefbe, 0xdfdd, 0xcffc, 0xbf1b, 0xaf3a, 0x9f59, 0x8f78,
-    0x9188, 0x81a9, 0xb1ca, 0xa1eb, 0xd10c, 0xc12d, 0xf14e, 0xe16f,
-    0x1080, 0x00a1, 0x30c2, 0x20e3, 0x5004, 0x4025, 0x7046, 0x6067,
-    0x83b9, 0x9398, 0xa3fb, 0xb3da, 0xc33d, 0xd31c, 0xe37f, 0xf35e,
-    0x02b1, 0x1290, 0x22f3, 0x32d2, 0x4235, 0x5214, 0x6277, 0x7256,
-    0xb5ea, 0xa5cb, 0x95a8, 0x8589, 0xf56e, 0xe54f, 0xd52c, 0xc50d,
-    0x34e2, 0x24c3, 0x14a0, 0x0481, 0x7466, 0x6447, 0x5424, 0x4405,
-    0xa7db, 0xb7fa, 0x8799, 0x97b8, 0xe75f, 0xf77e, 0xc71d, 0xd73c,
-    0x26d3, 0x36f2, 0x0691, 0x16b0, 0x6657, 0x7676, 0x4615, 0x5634,
-    0xd94c, 0xc96d, 0xf90e, 0xe92f, 0x99c8, 0x89e9, 0xb98a, 0xa9ab,
-    0x5844, 0x4865, 0x7806, 0x6827, 0x18c0, 0x08e1, 0x3882, 0x28a3,
-    0xcb7d, 0xdb5c, 0xeb3f, 0xfb1e, 0x8bf9, 0x9bd8, 0xabbb, 0xbb9a,
-    0x4a75, 0x5a54, 0x6a37, 0x7a16, 0x0af1, 0x1ad0, 0x2ab3, 0x3a92,
-    0xfd2e, 0xed0f, 0xdd6c, 0xcd4d, 0xbdaa, 0xad8b, 0x9de8, 0x8dc9,
-    0x7c26, 0x6c07, 0x5c64, 0x4c45, 0x3ca2, 0x2c83, 0x1ce0, 0x0cc1,
-    0xef1f, 0xff3e, 0xcf5d, 0xdf7c, 0xaf9b, 0xbfba, 0x8fd9, 0x9ff8,
-    0x6e17, 0x7e36, 0x4e55, 0x5e74, 0x2e93, 0x3eb2, 0x0ed1, 0x1ef0,
+const crc16tab: [256]u16 = tab: {
+    @setEvalBranchQuota(10000);
+
+    // CRC-16-CCITT/XMODEM
+    const poly: u32 = 0x1021;
+    var table: [256]u16 = undefined;
+
+    for (table) |*crc, i| {
+        crc.* = @as(u16, i) << 8;
+        var j = 0;
+        while (j < 8) : (j += 1) {
+            if (crc.* >> 15 != 0) {
+                crc.* = (crc.* << 1) ^ poly;
+            } else {
+                crc.* <<= 1;
+            }
+        }
+    }
+
+    break :tab table;
 };
 
 pub fn update(crc: u16, with_data: []const u8) u16 {
     var new_crc = crc;
     for (with_data) |b| {
-        new_crc = ((new_crc << 8) & 0xffff) ^ crc16tab[((new_crc >> 8) ^ @as(u16, b)) & 0x00ff];
+        new_crc = (new_crc << 8) ^ crc16tab[@truncate(u8, new_crc >> 8) ^ b];
     }
     return new_crc;
 }
@@ -51,7 +37,5 @@ pub fn make(data: []const u8) u16 {
 
 // validate will check the calculated CRC16 checksum for data against the expected.
 pub fn validate(data: []const u8, expected: u16) !void {
-    if (make(data) != expected) {
-        return error.InvalidChecksum;
-    }
+    if (make(data) != expected) return error.InvalidChecksum;
 }
-- 
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