jsonparse: A bunch of performance improvements

Turrns out JSON::XS had some pretty good ideas that I could borrow.

c/common.c

@@ -2,29 +2,47 @@
 
 typedef struct {
     SV *sv;
+    SV *mortal;
     char *cur;
     char *end;
     size_t maxlen;
+    int setutf8;
+    char sbuf[4096];
 } fustr;
 
-/* sv must be a new SV with a preallocated buffer */
-static void fustr_init_(pTHX_ fustr *s, SV *sv, size_t maxlen) {
-    s->sv = sv;
-    SvPOK_only(s->sv);
-    s->cur = SvPVX(s->sv);
-    s->end = SvEND(s->sv);
+static void fustr_init_(pTHX_ fustr *s, SV *mortal, size_t maxlen) {
+    s->sv = NULL;
+    s->cur = s->sbuf;
+    s->end = s->sbuf + (maxlen > sizeof s->sbuf ? sizeof s->sbuf : maxlen);
     s->maxlen = maxlen;
+    s->mortal = mortal;
+    s->setutf8 = 0;
 }
 
+#define fustr_start(s) (((s)->sv ? SvPVX((s)->sv) : (s)->sbuf))
+
 static void fustr_grow(pTHX_ fustr *s, size_t add) {
-    size_t off = s->cur - SvPVX(s->sv);
-    size_t newlen = 64;
+    size_t off = s->cur - (s->sv ? SvPVX(s->sv) : s->sbuf);
+    size_t newlen = sizeof s->sbuf;
+    char *buf;
     add += off;
     if (add > s->maxlen) croak("maximum string length exceeded");
     /* Increment to next power of two; SvGROW's default strategy is slow */
     while (newlen < add) newlen <<= 1;
     if (newlen > s->maxlen) newlen = s->maxlen;
-    char *buf = SvGROW(s->sv, newlen);
+    if (s->sv) {
+        buf = SvGROW(s->sv, newlen);
+    } else {
+        if (s->mortal) {
+            s->sv = s->mortal;
+            sv_setpv_bufsize(s->sv, off, newlen);
+        } else {
+            s->sv = newSV(newlen);
+        }
+        SvPOK_only(s->sv);
+        buf = SvPVX(s->sv);
+        memcpy(buf, s->sbuf, off);
+    }
     s->cur = buf + off;
     s->end = buf + (SvLEN(s->sv) > s->maxlen ? s->maxlen : SvLEN(s->sv));
 }
@@ -39,6 +57,11 @@ static inline void fustr_write_(pTHX_ fustr *s, const char *str, size_t n) {
     s->cur += n;
 }
 
+static inline void fustr_write_ch_(pTHX_ fustr *s, char x) {
+    fustr_reserve_(aTHX_ s, 1);
+    *(s->cur++) = x;
+}
+
 /* Adds n uninitialized bytes to the string and returns a buffer to write the data to */
 static inline char *fustr_write_buf_(pTHX_ fustr *s, size_t n) {
     fustr_reserve_(aTHX_ s, n);
@@ -50,13 +73,19 @@ static inline char *fustr_write_buf_(pTHX_ fustr *s, size_t n) {
 static SV *fustr_done_(pTHX_ fustr *s) {
     fustr_reserve_(aTHX_ s, 1);
     *s->cur = 0;
-    SvCUR_set(s->sv, s->cur - SvPVX(s->sv));
-    // TODO: SvPV_shrink_to_cur?
+    if (s->sv) {
+        SvCUR_set(s->sv, s->cur - SvPVX(s->sv));
+        // TODO: SvPV_shrink_to_cur?
+    } else {
+        s->sv = newSVpvn_flags(s->sbuf, s->cur - s->sbuf, s->mortal ? SVs_TEMP : 0);
+    }
+    if (s->setutf8) SvUTF8_on(s->sv);
     return s->sv;
 }
 
 #define fustr_init(a,b,c) fustr_init_(aTHX_ a,b,c)
 #define fustr_reserve(a,b) fustr_reserve_(aTHX_ a,b)
 #define fustr_write(a,b,c) fustr_write_(aTHX_ a,b,c)
+#define fustr_write_ch(a,b) fustr_write_ch_(aTHX_ a,b)
 #define fustr_write_buf(a,b) fustr_write_buf_(aTHX_ a,b)
 #define fustr_done(a) fustr_done_(aTHX_ a)