add fmt_utf8_scratch

fmt.h

@@ -196,6 +196,8 @@ size_t fmt_asn1derlength_scratch(char* dest,unsigned long long l) noexcept;	/* s
 att_write(1)
 size_t fmt_asn1dertag(char* dest,unsigned long long l) noexcept;	/* 1 byte for each 7 bits; upper bit = more bytes coming */
 
+// This function is only available on x86 and x86_64 (because unaligned
+// word write is allowed and does not cost performance):
 att_write(1)
 size_t fmt_asn1dertag_scratch(char* dest,unsigned long long l) noexcept;/* same, but is allowed to overwrite up to 7 additional bytes */
 

fmt/fmt_utf8.c

@@ -1,4 +1,4 @@
-#include "fmt.h"
+#include <libowfat/fmt.h>
 
 size_t fmt_utf8(char *dest,uint32_t n) {
   size_t i,j;
@@ -6,9 +6,8 @@ size_t fmt_utf8(char *dest,uint32_t n) {
     if (dest) *dest=(char)n;
     return 1;
   }
-  for (i=0x3f,j=1; i<=0x7fffffff; i=(i<<5)|0x1f, ++j) {
+  for (i=0x3f,j=0; i<=0x7fffffff; i=(i<<5)|0x1f, ++j) {
     if (i>=n) {
-      --j;
       if (dest) {
 	size_t k=j*6;
 	// gcc -fanalyze warns here that j-1 might underflow, leading to
@@ -30,4 +29,43 @@ size_t fmt_utf8(char *dest,uint32_t n) {
   return 0;
 }
 
+#if defined(__i386__) || defined(__x86_64__)
+
+// on architectures where unaligned writes are ok and don't carry a
+// performance pentalty, it may make sense to not write each byte
+// individually but write a 32-bit word at the end. The downside is that
+// this will write scratch bytes over the bytes following the utf-8. If
+// you allocated a few more bytes scratch space, and did not expect
+// those bytes to be untouched, you can use this function.
+// I did a small benchmark:
+//   for (i=j=0; i<1000; i+=fmt_utf8(buf+i,i));
+// Regular fmt_utf8: 13k cpu cycles, fmt_utf_scratch: 11k cpu cycles.
+
+size_t fmt_utf8_scratch(char *dest,uint32_t n) {
+  size_t i;
+  unsigned int j;
+  if (n<=0x7f) {
+    if (dest) *dest=(char)n;
+    return 1;
+  }
+  for (i=0x3f,j=0; i<=0x7fffffff; i=(i<<5)|0x1f, ++j) {
+    if (i>=n) {
+      if (dest) {
+	uint64_t buf=0;
+	while (n>0x3f) {
+	  buf = (buf << 8) + (n & 0x3f) + 0x80;
+	  n >>= 6;
+	}
+	buf = (buf << 8) + (unsigned char)((char)0xc0 >> (j-1)) + n;
+	*(uint64_t*)dest = buf;
+      }
+      return j+1;
+    }
+    if (i==0x7fffffff) return 0;
+  }
+  /* we were asked to encode a value that cannot be encoded */
+  return 0;
+}
+#endif
+
 /* unit tested via scan/scan_utf8.c */