jsonfmt: Add pretty option

That completes the json_format() function for now. At least, it now does
everything I had planned for it.

Ended up at a bit over 300 LOC. That's larger than I had expected, but
still alright.

FU/Util.pod

@@ -51,13 +51,20 @@ The following C<%options> are supported:
 
 =item canonical
 
-When set to a true value, write hash keys in deterministic (sorted) order. This
-option currently has no effect on tied hashes.
+Boolean, write hash keys in deterministic (sorted) order. This option currently
+has no effect on tied hashes.
+
+=item pretty
+
+Boolean, format JSON with newlines and indentation for easier reading.  Beauty
+is in the eye of the beholder, this option currently follows the convention
+used by L<JSON::XS> and others: 3 space indent and one space around the C<:>
+separating object keys and values. The exact format might change in later
+versions.
 
 =item utf8
 
-When set to a true value, returns a UTF-8 encoded byte string instead of a Perl
-Unicode string.
+Boolean, returns a UTF-8 encoded byte string instead of a Perl Unicode string.
 
 =item max_size
 

c/jsonfmt.c

@@ -2,10 +2,19 @@ typedef struct {
     fustr out;
     UV depth;
     int canon;
+    int pretty; /* <0 when disabled, current nesting level otherwise */
 } fujson_fmt_ctx;
 
 static void fujson_fmt(pTHX_ fujson_fmt_ctx *, SV *);
 
+static void fujson_fmt_indent(pTHX_ fujson_fmt_ctx *ctx) {
+    if (ctx->pretty >= 0) {
+        char *buf = fustr_write_buf(&ctx->out, 1 + ctx->pretty*3);
+        *buf = '\n';
+        memset(buf+1, ' ', ctx->pretty*3);
+    }
+}
+
 static void fujson_fmt_str(pTHX_ fujson_fmt_ctx *ctx, const char *stri, size_t len, int utf8) {
     size_t off = 0, loff;
     const unsigned char *str = (const unsigned char *)stri;
@@ -118,12 +127,16 @@ static void fujson_fmt_int(pTHX_ fujson_fmt_ctx *ctx, SV *val) {
 static void fujson_fmt_av(pTHX_ fujson_fmt_ctx *ctx, AV *av) {
     int i, len = av_count(av);
     fustr_write(&ctx->out, "[", 1);
+    ctx->pretty++;
     for (i=0; i<len; i++) {
         if (i) fustr_write(&ctx->out, ",", 1);
+        fujson_fmt_indent(aTHX_ ctx);
         SV **sv = av_fetch(av, i, 0);
         if (sv) fujson_fmt(aTHX_ ctx, *sv); /* sv will have magic if av is tied, but fujson_fmt() handles that. */
         else fustr_write(&ctx->out, "null", 4);
     }
+    ctx->pretty--;
+    if (i) fujson_fmt_indent(aTHX_ ctx);
     fustr_write(&ctx->out, "]", 1);
 }
 
@@ -148,9 +161,11 @@ static int fujson_fmt_hvcmp(const void *pa, const void *pb) {
 static void fujson_fmt_hvkv(pTHX_ fujson_fmt_ctx *ctx, HV *hv, HE *he, char **hestr) {
     STRLEN helen;
     if (*hestr) fustr_write(&ctx->out, ",", 1);
+    fujson_fmt_indent(aTHX_ ctx);
     *hestr = HePV(he, helen);
     fujson_fmt_str(aTHX_ ctx, *hestr, helen, HeUTF8(he));
-    fustr_write(&ctx->out, ":", 1);
+    if (ctx->pretty > 0) fustr_write(&ctx->out, " : ", 3);
+    else fustr_write(&ctx->out, ":", 1);
     fujson_fmt(aTHX_ ctx, UNLIKELY(SvMAGICAL(hv)) ? hv_iterval(hv, he) : HeVAL(he));
 }
 
@@ -160,6 +175,7 @@ static void fujson_fmt_hv(pTHX_ fujson_fmt_ctx *ctx, HV *hv) {
 
     int numkeys = hv_iterinit(hv);
     fustr_write(&ctx->out, "{", 1);
+    ctx->pretty++;
 
     /* Canonical order on tied hashes is not supported. Cpanel::JSON::XS has
      * code to deal with that case and it's absolutely horrifying. */
@@ -187,6 +203,8 @@ static void fujson_fmt_hv(pTHX_ fujson_fmt_ctx *ctx, HV *hv) {
     } else {
         while ((he = hv_iternext(hv))) fujson_fmt_hvkv(aTHX_ ctx, hv, he, &hestr);
     }
+    ctx->pretty--;
+    if (hestr) fujson_fmt_indent(aTHX_ ctx);
     fustr_write(&ctx->out, "}", 1);
 }
 
@@ -269,6 +287,7 @@ static SV *fujson_fmt_xs(pTHX_ I32 ax, I32 argc, SV *val) {
 
     ctx.out.maxlen = 0;
     ctx.depth = 0;
+    ctx.pretty = INT_MIN;
     ctx.canon = 0;
     while (i < argc) {
         arg = SvPV_nolen(ST(i));
@@ -278,6 +297,7 @@ static SV *fujson_fmt_xs(pTHX_ I32 ax, I32 argc, SV *val) {
         i++;
 
         if (strcmp(arg, "canonical") == 0) ctx.canon = SvPVXtrue(r);
+        else if (strcmp(arg, "pretty") == 0) ctx.pretty = SvPVXtrue(r) ? 0 : INT_MIN;
         else if (strcmp(arg, "utf8") == 0) encutf8 = SvPVXtrue(r);
         else if (strcmp(arg, "max_size") == 0) ctx.out.maxlen = SvUV(r);
         else if (strcmp(arg, "max_depth") == 0) ctx.depth = SvUV(r);
@@ -288,9 +308,8 @@ static SV *fujson_fmt_xs(pTHX_ I32 ax, I32 argc, SV *val) {
 
     fustr_init(&ctx.out, 128, ctx.out.maxlen);
     fujson_fmt(aTHX_ &ctx, val);
+    if (ctx.pretty >= 0) fustr_write(&ctx.out, "\n", 1);
     r = fustr_done(&ctx.out);
     if (!encutf8) SvUTF8_on(r);
     return r;
 }
-
-/* TODO: pretty */

t/json_format.t

@@ -68,7 +68,7 @@ my @errors = (
     do { my $o = {}; bless $o, 'MyToJSONSelf' }, qr/MyToJSONSelf::TO_JSON method returned same object as was passed instead of a new one/,
 );
 
-plan tests => @tests*2 + @errors/2 + 9;
+plan tests => @tests*2 + @errors/2 + 10;
 
 for my($in, $exp) (@tests) {
     my $out = json_format $in;
@@ -90,6 +90,28 @@ for my ($in, $exp) (@errors) {
 is json_format({qw/a 1 b 2 c 3 d 4 d1 5 d11 6/, do { use utf8; qw/ü 7 月 8 💩 9/ }}, canonical => 1),
    do { use utf8; '{"a":"1","b":"2","c":"3","d":"4","d1":"5","d11":"6","ü":"7","月":"8","💩":"9"}' };
 
+is json_format(
+      { a => [], b => {}, c => { x => 1 }, d => { y => true, z => false }, e => [1,2,3] },
+      canonical => 1, pretty => 1
+    ), <<_;
+{
+   "a" : [],
+   "b" : {},
+   "c" : {
+      "x" : 1
+   },
+   "d" : {
+      "y" : true,
+      "z" : false
+   },
+   "e" : [
+      1,
+      2,
+      3
+   ]
+}
+_
+
 
 eval { json_format [[]], max_depth => 2 };
 like $@, qr/max_depth exceeded while formatting JSON/;