json_writer: new routines to create JSON data

Add "struct json_writer" and a series of jw_ routines to compose JSON
data into a string buffer.  The resulting string may then be printed by
commands wanting to support a JSON-like output format.

The json_writer is limited to correctly formatting structured data for
output.  It does not attempt to build an object model of the JSON data.

We say "JSON-like" because we do not enforce the Unicode (usually UTF-8)
requirement on string fields.  Internally, Git does not necessarily have
Unicode/UTF-8 data for most fields, so it is currently unclear the best
way to enforce that requirement.  For example, on Linux pathnames can
contain arbitrary 8-bit character data, so a command like "status" would
not know how to encode the reported pathnames.  We may want to revisit
this (or double encode such strings) in the future.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Helped-by: René Scharfe <l.s.r@web.de>
Helped-by: Wink Saville <wink@saville.com>
Helped-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 414 insertions, 0 deletions

diff --git a/json-writer.c b/json-writer.c
new file mode 100644
index 0000000..aadb9db
--- /dev/null
+++ b/json-writer.c
@@ -0,0 +1,414 @@
+#include "cache.h"
+#include "json-writer.h"
+
+void jw_init(struct json_writer *jw)
+{
+	strbuf_init(&jw->json, 0);
+	strbuf_init(&jw->open_stack, 0);
+	jw->need_comma = 0;
+	jw->pretty = 0;
+}
+
+void jw_release(struct json_writer *jw)
+{
+	strbuf_release(&jw->json);
+	strbuf_release(&jw->open_stack);
+}
+
+/*
+ * Append JSON-quoted version of the given string to 'out'.
+ */
+static void append_quoted_string(struct strbuf *out, const char *in)
+{
+	unsigned char c;
+
+	strbuf_addch(out, '"');
+	while ((c = *in++) != '\0') {
+		if (c == '"')
+			strbuf_addstr(out, "\\\"");
+		else if (c == '\\')
+			strbuf_addstr(out, "\\\\");
+		else if (c == '\n')
+			strbuf_addstr(out, "\\n");
+		else if (c == '\r')
+			strbuf_addstr(out, "\\r");
+		else if (c == '\t')
+			strbuf_addstr(out, "\\t");
+		else if (c == '\f')
+			strbuf_addstr(out, "\\f");
+		else if (c == '\b')
+			strbuf_addstr(out, "\\b");
+		else if (c < 0x20)
+			strbuf_addf(out, "\\u%04x", c);
+		else
+			strbuf_addch(out, c);
+	}
+	strbuf_addch(out, '"');
+}
+
+static void indent_pretty(struct json_writer *jw)
+{
+	int k;
+
+	for (k = 0; k < jw->open_stack.len; k++)
+		strbuf_addstr(&jw->json, "  ");
+}
+
+/*
+ * Begin an object or array (either top-level or nested within the currently
+ * open object or array).
+ */
+static void begin(struct json_writer *jw, char ch_open, int pretty)
+{
+	jw->pretty = pretty;
+
+	strbuf_addch(&jw->json, ch_open);
+
+	strbuf_addch(&jw->open_stack, ch_open);
+	jw->need_comma = 0;
+}
+
+/*
+ * Assert that the top of the open-stack is an object.
+ */
+static void assert_in_object(const struct json_writer *jw, const char *key)
+{
+	if (!jw->open_stack.len)
+		BUG("json-writer: object: missing jw_object_begin(): '%s'", key);
+	if (jw->open_stack.buf[jw->open_stack.len - 1] != '{')
+		BUG("json-writer: object: not in object: '%s'", key);
+}
+
+/*
+ * Assert that the top of the open-stack is an array.
+ */
+static void assert_in_array(const struct json_writer *jw)
+{
+	if (!jw->open_stack.len)
+		BUG("json-writer: array: missing jw_array_begin()");
+	if (jw->open_stack.buf[jw->open_stack.len - 1] != '[')
+		BUG("json-writer: array: not in array");
+}
+
+/*
+ * Add comma if we have already seen a member at this level.
+ */
+static void maybe_add_comma(struct json_writer *jw)
+{
+	if (jw->need_comma)
+		strbuf_addch(&jw->json, ',');
+	else
+		jw->need_comma = 1;
+}
+
+static void fmt_double(struct json_writer *jw, int precision,
+			      double value)
+{
+	if (precision < 0) {
+		strbuf_addf(&jw->json, "%f", value);
+	} else {
+		struct strbuf fmt = STRBUF_INIT;
+		strbuf_addf(&fmt, "%%.%df", precision);
+		strbuf_addf(&jw->json, fmt.buf, value);
+		strbuf_release(&fmt);
+	}
+}
+
+static void object_common(struct json_writer *jw, const char *key)
+{
+	assert_in_object(jw, key);
+	maybe_add_comma(jw);
+
+	if (jw->pretty) {
+		strbuf_addch(&jw->json, '\n');
+		indent_pretty(jw);
+	}
+
+	append_quoted_string(&jw->json, key);
+	strbuf_addch(&jw->json, ':');
+	if (jw->pretty)
+		strbuf_addch(&jw->json, ' ');
+}
+
+static void array_common(struct json_writer *jw)
+{
+	assert_in_array(jw);
+	maybe_add_comma(jw);
+
+	if (jw->pretty) {
+		strbuf_addch(&jw->json, '\n');
+		indent_pretty(jw);
+	}
+}
+
+/*
+ * Assert that the given JSON object or JSON array has been properly
+ * terminated.  (Has closing bracket.)
+ */
+static void assert_is_terminated(const struct json_writer *jw)
+{
+	if (jw->open_stack.len)
+		BUG("json-writer: object: missing jw_end(): '%s'",
+		    jw->json.buf);
+}
+
+void jw_object_begin(struct json_writer *jw, int pretty)
+{
+	begin(jw, '{', pretty);
+}
+
+void jw_object_string(struct json_writer *jw, const char *key, const char *value)
+{
+	object_common(jw, key);
+	append_quoted_string(&jw->json, value);
+}
+
+void jw_object_intmax(struct json_writer *jw, const char *key, intmax_t value)
+{
+	object_common(jw, key);
+	strbuf_addf(&jw->json, "%"PRIdMAX, value);
+}
+
+void jw_object_double(struct json_writer *jw, const char *key, int precision,
+		      double value)
+{
+	object_common(jw, key);
+	fmt_double(jw, precision, value);
+}
+
+void jw_object_true(struct json_writer *jw, const char *key)
+{
+	object_common(jw, key);
+	strbuf_addstr(&jw->json, "true");
+}
+
+void jw_object_false(struct json_writer *jw, const char *key)
+{
+	object_common(jw, key);
+	strbuf_addstr(&jw->json, "false");
+}
+
+void jw_object_bool(struct json_writer *jw, const char *key, int value)
+{
+	if (value)
+		jw_object_true(jw, key);
+	else
+		jw_object_false(jw, key);
+}
+
+void jw_object_null(struct json_writer *jw, const char *key)
+{
+	object_common(jw, key);
+	strbuf_addstr(&jw->json, "null");
+}
+
+static void increase_indent(struct strbuf *sb,
+			    const struct json_writer *jw,
+			    int indent)
+{
+	int k;
+
+	strbuf_reset(sb);
+	for (k = 0; k < jw->json.len; k++) {
+		char ch = jw->json.buf[k];
+		strbuf_addch(sb, ch);
+		if (ch == '\n')
+			strbuf_addchars(sb, ' ', indent);
+	}
+}
+
+static void kill_indent(struct strbuf *sb,
+			const struct json_writer *jw)
+{
+	int k;
+	int eat_it = 0;
+
+	strbuf_reset(sb);
+	for (k = 0; k < jw->json.len; k++) {
+		char ch = jw->json.buf[k];
+		if (eat_it && ch == ' ')
+			continue;
+		if (ch == '\n') {
+			eat_it = 1;
+			continue;
+		}
+		eat_it = 0;
+		strbuf_addch(sb, ch);
+	}
+}
+
+static void append_sub_jw(struct json_writer *jw,
+			  const struct json_writer *value)
+{
+	/*
+	 * If both are pretty, increase the indentation of the sub_jw
+	 * to better fit under the super.
+	 *
+	 * If the super is pretty, but the sub_jw is compact, leave the
+	 * sub_jw compact.  (We don't want to parse and rebuild the sub_jw
+	 * for this debug-ish feature.)
+	 *
+	 * If the super is compact, and the sub_jw is pretty, convert
+	 * the sub_jw to compact.
+	 *
+	 * If both are compact, keep the sub_jw compact.
+	 */
+	if (jw->pretty && jw->open_stack.len && value->pretty) {
+		struct strbuf sb = STRBUF_INIT;
+		increase_indent(&sb, value, jw->open_stack.len * 2);
+		strbuf_addbuf(&jw->json, &sb);
+		strbuf_release(&sb);
+		return;
+	}
+	if (!jw->pretty && value->pretty) {
+		struct strbuf sb = STRBUF_INIT;
+		kill_indent(&sb, value);
+		strbuf_addbuf(&jw->json, &sb);
+		strbuf_release(&sb);
+		return;
+	}
+
+	strbuf_addbuf(&jw->json, &value->json);
+}
+
+/*
+ * Append existing (properly terminated) JSON sub-data (object or array)
+ * as-is onto the given JSON data.
+ */
+void jw_object_sub_jw(struct json_writer *jw, const char *key,
+		      const struct json_writer *value)
+{
+	assert_is_terminated(value);
+
+	object_common(jw, key);
+	append_sub_jw(jw, value);
+}
+
+void jw_object_inline_begin_object(struct json_writer *jw, const char *key)
+{
+	object_common(jw, key);
+
+	jw_object_begin(jw, jw->pretty);
+}
+
+void jw_object_inline_begin_array(struct json_writer *jw, const char *key)
+{
+	object_common(jw, key);
+
+	jw_array_begin(jw, jw->pretty);
+}
+
+void jw_array_begin(struct json_writer *jw, int pretty)
+{
+	begin(jw, '[', pretty);
+}
+
+void jw_array_string(struct json_writer *jw, const char *value)
+{
+	array_common(jw);
+	append_quoted_string(&jw->json, value);
+}
+
+void jw_array_intmax(struct json_writer *jw, intmax_t value)
+{
+	array_common(jw);
+	strbuf_addf(&jw->json, "%"PRIdMAX, value);
+}
+
+void jw_array_double(struct json_writer *jw, int precision, double value)
+{
+	array_common(jw);
+	fmt_double(jw, precision, value);
+}
+
+void jw_array_true(struct json_writer *jw)
+{
+	array_common(jw);
+	strbuf_addstr(&jw->json, "true");
+}
+
+void jw_array_false(struct json_writer *jw)
+{
+	array_common(jw);
+	strbuf_addstr(&jw->json, "false");
+}
+
+void jw_array_bool(struct json_writer *jw, int value)
+{
+	if (value)
+		jw_array_true(jw);
+	else
+		jw_array_false(jw);
+}
+
+void jw_array_null(struct json_writer *jw)
+{
+	array_common(jw);
+	strbuf_addstr(&jw->json, "null");
+}
+
+void jw_array_sub_jw(struct json_writer *jw, const struct json_writer *value)
+{
+	assert_is_terminated(value);
+
+	array_common(jw);
+	append_sub_jw(jw, value);
+}
+
+void jw_array_argc_argv(struct json_writer *jw, int argc, const char **argv)
+{
+	int k;
+
+	for (k = 0; k < argc; k++)
+		jw_array_string(jw, argv[k]);
+}
+
+void jw_array_argv(struct json_writer *jw, const char **argv)
+{
+	while (*argv)
+		jw_array_string(jw, *argv++);
+}
+
+void jw_array_inline_begin_object(struct json_writer *jw)
+{
+	array_common(jw);
+
+	jw_object_begin(jw, jw->pretty);
+}
+
+void jw_array_inline_begin_array(struct json_writer *jw)
+{
+	array_common(jw);
+
+	jw_array_begin(jw, jw->pretty);
+}
+
+int jw_is_terminated(const struct json_writer *jw)
+{
+	return !jw->open_stack.len;
+}
+
+void jw_end(struct json_writer *jw)
+{
+	char ch_open;
+	int len;
+
+	if (!jw->open_stack.len)
+		BUG("json-writer: too many jw_end(): '%s'", jw->json.buf);
+
+	len = jw->open_stack.len - 1;
+	ch_open = jw->open_stack.buf[len];
+
+	strbuf_setlen(&jw->open_stack, len);
+	jw->need_comma = 1;
+
+	if (jw->pretty) {
+		strbuf_addch(&jw->json, '\n');
+		indent_pretty(jw);
+	}
+
+	if (ch_open == '{')
+		strbuf_addch(&jw->json, '}');
+	else
+		strbuf_addch(&jw->json, ']');
+}