json_writer: new routines to create data in JSON formatjh/json-writer

Add a series of jw_ routines and "struct json_writer" structure to compose
JSON data.  The resulting string data can then be output by commands wanting
to support a JSON output format.

The json-writer routines can be used to generate structured data in a
JSON-like format.  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 Linx
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.

The initial use for the json-writer routines is for generating telemetry
data for executed Git commands.  Later, we may want to use them in other
commands, such as status.

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, 91 insertions, 0 deletions

diff --git a/json-writer.h b/json-writer.h
new file mode 100644
index 0000000000..7d07dc6da8
--- /dev/null
+++ b/json-writer.h
@@ -0,0 +1,91 @@
+#ifndef JSON_WRITER_H
+#define JSON_WRITER_H
+
+/*
+ * JSON data structures are defined at:
+ * [1] http://www.ietf.org/rfc/rfc7159.txt
+ * [2] http://json.org/
+ *
+ * The JSON-writer API allows one to build JSON data structures using a
+ * simple wrapper around a "struct strbuf" buffer.  It is intended as a
+ * simple API to build output strings; it is not intended to be a general
+ * object model for JSON data.  In particular, it does not re-order keys
+ * in an object (dictionary), it does not de-dup keys in an object, and
+ * it does not allow lookup or parsing of JSON data.
+ *
+ * All string values (both keys and string r-values) are properly quoted
+ * and escaped if they contain special characters.
+ *
+ * These routines create compact JSON data (with no unnecessary whitespace,
+ * newlines, or indenting).  If you get an unexpected response, verify
+ * that you're not expecting a pretty JSON string.
+ *
+ * Both "JSON objects" (aka sets of k/v pairs) and "JSON array" can be
+ * constructed using a 'begin append* end' model.
+ *
+ * Nested objects and arrays can either be constructed bottom up (by
+ * creating sub object/arrays first and appending them to the super
+ * object/array) -or- by building them inline in one pass.  This is a
+ * personal style and/or data shape choice.
+ *
+ * See t/helper/test-json-writer.c for various usage examples.
+ *
+ * LIMITATIONS:
+ * ============
+ *
+ * The JSON specification [1,2] defines string values as Unicode data
+ * and probably UTF-8 encoded.  The current json-writer API does not
+ * enforce this and will write any string as received.  However, it will
+ * properly quote and backslash-escape them as necessary.  It is up to
+ * the caller to UTF-8 encode their strings *before* passing them to this
+ * API.  This layer should not have to try to guess the encoding or locale
+ * of the given strings.
+ */
+
+struct json_writer
+{
+	struct strbuf json;
+	struct strbuf open_stack;
+	int first;
+	int pretty;
+};
+
+#define JSON_WRITER_INIT { STRBUF_INIT, STRBUF_INIT, 0, 0 }
+
+void jw_init(struct json_writer *jw);
+void jw_release(struct json_writer *jw);
+
+void jw_object_begin(struct json_writer *jw, int pretty);
+void jw_array_begin(struct json_writer *jw, int pretty);
+
+void jw_object_string(struct json_writer *jw, const char *key,
+		      const char *value);
+void jw_object_intmax(struct json_writer *jw, const char *key, intmax_t value);
+void jw_object_double(struct json_writer *jw, const char *key, int precision,
+		      double value);
+void jw_object_true(struct json_writer *jw, const char *key);
+void jw_object_false(struct json_writer *jw, const char *key);
+void jw_object_null(struct json_writer *jw, const char *key);
+void jw_object_sub_jw(struct json_writer *jw, const char *key,
+		      const struct json_writer *value);
+
+void jw_object_inline_begin_object(struct json_writer *jw, const char *key);
+void jw_object_inline_begin_array(struct json_writer *jw, const char *key);
+
+void jw_array_string(struct json_writer *jw, const char *value);
+void jw_array_intmax(struct json_writer *jw, intmax_t value);
+void jw_array_double(struct json_writer *jw, int precision, double value);
+void jw_array_true(struct json_writer *jw);
+void jw_array_false(struct json_writer *jw);
+void jw_array_null(struct json_writer *jw);
+void jw_array_sub_jw(struct json_writer *jw, const struct json_writer *value);
+void jw_array_argc_argv(struct json_writer *jw, int argc, const char **argv);
+void jw_array_argv(struct json_writer *jw, const char **argv);
+
+void jw_array_inline_begin_object(struct json_writer *jw);
+void jw_array_inline_begin_array(struct json_writer *jw);
+
+int jw_is_terminated(const struct json_writer *jw);
+void jw_end(struct json_writer *jw);
+
+#endif /* JSON_WRITER_H */