Add CJK codecs support as discussed on python-dev. (SF #873597)

Several style fixes are suggested by Martin v. Loewis and
Marc-Andre Lemburg. Thanks!
diff --git a/Modules/cjkcodecs/_gb18030.c b/Modules/cjkcodecs/_gb18030.c
new file mode 100644
index 0000000..f0bc2f0
--- /dev/null
+++ b/Modules/cjkcodecs/_gb18030.c
@@ -0,0 +1,181 @@
+/*
+ * _gb18030.c: the GB18030 codec
+ *
+ * Written by Hye-Shik Chang <perky@FreeBSD.org>
+ * $CJKCodecs: _gb18030.c,v 1.2 2003/12/31 05:46:55 perky Exp $
+ */
+
+#include "codeccommon.h"
+#include "tweak_gbk.h"
+#include "map_gb18030uni.h"
+
+ENCMAP(gbcommon)
+ENCMAP(gb18030ext)
+DECMAP(gb2312)
+DECMAP(gbkext)
+DECMAP(gb18030ext)
+
+ENCODER(gb18030)
+{
+    while (inleft > 0) {
+        ucs4_t  c = IN1;
+        DBCHAR  code;
+
+        if (c < 0x80) {
+            WRITE1(c)
+            NEXT(1, 1)
+            continue;
+        }
+
+        DECODE_SURROGATE(c)
+        if (c > 0x10FFFF)
+#if Py_UNICODE_SIZE == 2
+            return 2; /* surrogates pair */
+#else
+            return 1;
+#endif
+        else if (c >= 0x10000) {
+            ucs4_t   tc = c - 0x10000;
+
+            RESERVE_OUTBUF(4)
+
+            OUT4((unsigned char)(tc % 10) + 0x30)
+            tc /= 10;
+            OUT3((unsigned char)(tc % 126) + 0x81)
+            tc /= 126;
+            OUT2((unsigned char)(tc % 10) + 0x30)
+            tc /= 10;
+            OUT1((unsigned char)(tc + 0x90))
+
+#if Py_UNICODE_SIZE == 2
+            NEXT(2, 4) /* surrogates pair */
+#else
+            NEXT(1, 4)
+#endif
+            continue;
+        }
+
+        RESERVE_OUTBUF(2)
+
+        GBK_PREENCODE(c, code)
+        else TRYMAP_ENC(gbcommon, code, c);
+        else TRYMAP_ENC(gb18030ext, code, c);
+        else {
+            const struct _gb18030_to_unibmp_ranges  *utrrange;
+
+            RESERVE_OUTBUF(4)
+
+            for (utrrange = gb18030_to_unibmp_ranges;
+                 utrrange->first != 0;
+                 utrrange++)
+                if (utrrange->first <= c && c <= utrrange->last) {
+                    Py_UNICODE   tc;
+
+                    tc = c - utrrange->first + utrrange->base;
+
+                    OUT4((unsigned char)(tc % 10) + 0x30)
+                    tc /= 10;
+                    OUT3((unsigned char)(tc % 126) + 0x81)
+                    tc /= 126;
+                    OUT2((unsigned char)(tc % 10) + 0x30)
+                    tc /= 10;
+                    OUT1((unsigned char)tc + 0x81)
+
+                    NEXT(1, 4)
+                    break;
+                }
+
+            if (utrrange->first == 0) {
+                PyErr_SetString(PyExc_RuntimeError,
+                                "unicode mapping invalid");
+                return 1;
+            }
+            continue;
+        }
+
+        OUT1((code >> 8) | 0x80)
+        if (code & 0x8000)
+            OUT2((code & 0xFF)) /* MSB set: GBK or GB18030ext */
+        else
+            OUT2((code & 0xFF) | 0x80) /* MSB unset: GB2312 */
+
+        NEXT(1, 2)
+    }
+
+    return 0;
+}
+
+DECODER(gb18030)
+{
+    while (inleft > 0) {
+        unsigned char    c = IN1, c2;
+
+        RESERVE_OUTBUF(1)
+
+        if (c < 0x80) {
+            OUT1(c)
+            NEXT(1, 1)
+            continue;
+        }
+
+        RESERVE_INBUF(2)
+
+        c2 = IN2;
+        if (c2 >= 0x30 && c2 <= 0x39) { /* 4 bytes seq */
+            const struct _gb18030_to_unibmp_ranges *utr;
+            unsigned char    c3, c4;
+            ucs4_t           lseq;
+
+            RESERVE_INBUF(4)
+            c3 = IN3;
+            c4 = IN4;
+            if (c < 0x81 || c3 < 0x81 || c4 < 0x30 || c4 > 0x39)
+                return 4;
+            c -= 0x81;  c2 -= 0x30;
+            c3 -= 0x81; c4 -= 0x30;
+
+            if (c < 4) { /* U+0080 - U+FFFF */
+                lseq = ((ucs4_t)c * 10 + c2) * 1260 +
+                        (ucs4_t)c3 * 10 + c4;
+                if (lseq < 39420) {
+                    for (utr = gb18030_to_unibmp_ranges;
+                         lseq >= (utr + 1)->base;
+                         utr++) ;
+                    OUT1(utr->first - utr->base + lseq)
+                    NEXT(4, 1)
+                    continue;
+                }
+            }
+            else if (c >= 15) { /* U+10000 - U+10FFFF */
+                lseq = 0x10000 + (((ucs4_t)c-15) * 10 + c2) * 1260 +
+                        (ucs4_t)c3 * 10 + c4;
+                if (lseq <= 0x10FFFF) {
+                    PUTUCS4(lseq);
+                    NEXT_IN(4)
+                    continue;
+                }
+            }
+            return 4;
+        }
+
+        GBK_PREDECODE(c, c2, **outbuf)
+        else TRYMAP_DEC(gb2312, **outbuf, c ^ 0x80, c2 ^ 0x80);
+        else TRYMAP_DEC(gbkext, **outbuf, c, c2);
+        else TRYMAP_DEC(gb18030ext, **outbuf, c, c2);
+        else return 2;
+
+        NEXT(2, 1)
+    }
+
+    return 0;
+}
+
+#include "codecentry.h"
+BEGIN_CODEC_REGISTRY(gb18030)
+    MAPOPEN(zh_CN)
+        IMPORTMAP_DEC(gb2312)
+        IMPORTMAP_DEC(gbkext)
+        IMPORTMAP_ENC(gbcommon)
+        IMPORTMAP_ENCDEC(gb18030ext)
+    MAPCLOSE()
+END_CODEC_REGISTRY(gb18030)