.c -> .cpp

[xonotic/gmqcc.git] / utf8.cpp

diff --git a/utf8.cpp b/utf8.cpp
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+++ b/utf8.cpp
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+#include "gmqcc.h"
+
+/*
+ * Based on the flexible and economical utf8 decoder:
+ * http://bjoern.hoehrmann.de/utf-8/decoder/dfa/
+ *
+ * This is slightly more economical, the fastest way to decode utf8 is
+ * with a lookup table as in:
+ *
+ * first 1-byte lookup
+ * if that fails, 2-byte lookup
+ * if that fails, 3-byte lookup
+ * if that fails, 4-byte lookup
+ *
+ * The following table can be generated with some interval trickery.
+ * consider an interval [a, b):
+ *
+ *      a must be 0x80 or b must be 0xc0, lower 3 bits
+ *      are clear, thus:
+ *          interval(a,b) = ((uint32_t)((a==0x80?0x40-b:-a)<<23))
+ *
+ * The failstate can be represented as interval(0x80,0x80), it's
+ * odd to see but this is a full state machine.
+ *
+ * The table than maps the corresponding sections as a serise of
+ * intervals.
+ *
+ * In this table the transition values are pre-multiplied with 16 to
+ * save a shift instruction for every byte, we throw away fillers
+ * which makes the table smaller.
+ *
+ * The first section of the table handles bytes with leading C
+ * The second section of the table handles bytes with leading D
+ * The third section of the table handles bytes with leading E
+ * The last section of the table handles bytes with leading F
+ *
+ * The values themselfs in the table are arranged so that when you
+ * left shift them by 6 to shift continuation characters into place, the
+ * new top bits tell you:
+ *
+ *  1 - if you keep going
+ *  2 - the range of valid values for the next byte
+ */
+static const uint32_t utf8_tab[] = {
+    0xC0000002, 0xC0000003, 0xC0000004, 0xC0000005, 0xC0000006,
+    0xC0000007, 0xC0000008, 0xC0000009, 0xC000000A, 0xC000000B,
+    0xC000000C, 0xC000000D, 0xC000000E, 0xC000000F, 0xC0000010,
+    0xC0000011, 0xC0000012, 0xC0000013, 0xC0000014, 0xC0000015,
+    0xC0000016, 0xC0000017, 0xC0000018, 0xC0000019, 0xC000001A,
+    0xC000001B, 0xC000001C, 0xC000001D, 0xC000001E, 0xC000001F,
+    0xB3000000, 0xC3000001, 0xC3000002, 0xC3000003, 0xC3000004,
+    0xC3000005, 0xC3000006, 0xC3000007, 0xC3000008, 0xC3000009,
+    0xC300000A, 0xC300000B, 0xC300000C, 0xD300000D, 0xC300000E,
+    0xC300000F, 0xBB0C0000, 0xC30C0001, 0xC30C0002, 0xC30C0003,
+    0xD30C0004
+};
+
+int utf8_from(char *s, utf8ch_t ch) {
+    if (!s)
+        return 0;
+
+    if ((unsigned)ch < 0x80) {
+        *s = ch;
+        return 1;
+    } else if ((unsigned)ch < 0x800) {
+        *s++ = 0xC0 | (ch >> 6);
+        *s   = 0x80 | (ch & 0x3F);
+        return 2;
+    } else if ((unsigned)ch < 0xD800 || (unsigned)ch - 0xE000 < 0x2000) {
+        *s++ = 0xE0 | (ch >> 12);
+        *s++ = 0x80 | ((ch >> 6) & 0x3F);
+        *s   = 0x80 | (ch & 0x3F);
+        return 3;
+    } else if ((unsigned)ch - 0x10000 < 0x100000) {
+        *s++ = 0xF0 | (ch >> 18);
+        *s++ = 0x80 | ((ch >> 12) & 0x3F);
+        *s++ = 0x80 | ((ch >> 6) & 0x3F);
+        *s   = 0x80 | (ch & 0x3F);
+        return 4;
+    }
+    return 0;
+}
+
+int utf8_to(utf8ch_t *i, const unsigned char *s, size_t n) {
+    unsigned c,j;
+
+    if (!s || !n)
+        return 0;
+
+    /* This is consistent with mbtowc behaviour. */
+    if (!i)
+        i = (utf8ch_t*)(void*)&i;
+
+    if (*s < 0x80)
+        return !!(*i = *s);
+    if (*s-0xC2U > 0x32)
+        return 0;
+
+    c = utf8_tab[*s++-0xC2U];
+
+    /*
+     * Avoid excessive checks against n.
+     *
+     * When shifting state `n-1` times does not clear the high bit,
+     * then the value of `n` won't satisfy the condition to read a
+     * character as it will be insufficent.
+     */
+    if (n < 4 && ((c<<(6*n-6)) & (1U << 31)))
+        return 0;
+
+    /*
+     * The upper 6 state bits are negitive integer offset to a bound-check
+     * next byte equivlant to: ((b-0x80)+(b+offset))&~0x3f
+     */
+    if ((((*s>>3)-0x10)|((*s>>3)+((int32_t)c>>26))) & ~7)
+        return 0;
+
+    for (j=2; j<3; j++) {
+        if (!((c = c<<6 | (*s++-0x80))&(1U<<31))) {
+            *i = c;
+            return j;
+        }
+        if (*s-0x80U >= 0x40)
+            return 0;
+    }
+
+    *i = c<<6 | (*s++-0x80);
+    return 4;
+}