# A simple ad-hoc JSON parser
#
# To parse a simple JSON document, read it as a String and give it to `parse_entity`
# NOTE: if your document contains several non-nested entities, use `parse_entity` for each
# JSON entity to parse
class JSONStringParser
	super StringProcessor

	# Parses a JSON Entity
	#
	# ~~~nit
	# var p = new JSONStringParser("""{"numbers": [1,23,3], "string": "string"}""")
	# assert p.parse_entity isa JsonObject
	# ~~~
	fun parse_entity: nullable Serializable do
		var srclen = len
		ignore_whitespaces
		if pos >= srclen then return make_parse_error("Empty JSON")
		var c = src[pos]
		if c == '[' then
			pos += 1
			return parse_json_array
		else if c == '"' then
			var s = parse_json_string
			return s
		else if c == '{' then
			pos += 1
			return parse_json_object
		else if c == 'f' then
			if pos + 4 >= srclen then make_parse_error("Error: bad JSON entity")
			if src[pos + 1] == 'a' and src[pos + 2] == 'l' and src[pos + 3] == 's' and src[pos + 4] == 'e' then
				pos += 5
				return false
			end
			return make_parse_error("Error: bad JSON entity")
		else if c == 't' then
			if pos + 3 >= srclen then make_parse_error("Error: bad JSON entity")
			if src[pos + 1] == 'r' and src[pos + 2] == 'u' and src[pos + 3] == 'e' then
				pos += 4
				return true
			end
			return make_parse_error("Error: bad JSON entity")
		else if c == 'n' then
			if pos + 3 >= srclen then make_parse_error("Error: bad JSON entity")
			if src[pos + 1] == 'u' and src[pos + 2] == 'l' and src[pos + 3] == 'l' then
				pos += 4
				return null
			end
			return make_parse_error("Error: bad JSON entity")
		end
		if not c.is_json_num_start then return make_parse_error("Bad JSON character")
		return parse_json_number
	end

	# Parses a JSON Array
	fun parse_json_array: Serializable do
		var max = len
		if pos >= max then return make_parse_error("Incomplete JSON array")
		var arr = new JsonArray
		var c = src[pos]
		while not c == ']' do
			ignore_whitespaces
			if pos >= max then return make_parse_error("Incomplete JSON array")
			if src[pos] == ']' then break
			var ent = parse_entity
			#print "Parsed an entity {ent} for a JSON array"
			if ent isa JsonParseError then return ent
			arr.add ent
			ignore_whitespaces
			if pos >= max then return make_parse_error("Incomplete JSON array")
			c = src[pos]
			if c == ']' then break
			if c != ',' then return make_parse_error("Bad array separator {c}")
			pos += 1
		end
		pos += 1
		return arr
	end

	# Parses a JSON Object
	fun parse_json_object: Serializable do
		var max = len
		if pos >= max then return make_parse_error("Incomplete JSON object")
		var obj = new JsonObject
		var c = src[pos]
		while not c == '}' do
			ignore_whitespaces
			if pos >= max then return make_parse_error("Malformed JSON object")
			if src[pos] == '}' then break
			var key = parse_entity
			#print "Parsed key {key} for JSON object"
			if not key isa String then return make_parse_error("Bad key format {key or else "null"}")
			ignore_whitespaces
			if pos >= max then return make_parse_error("Incomplete JSON object")
			if not src[pos] == ':' then return make_parse_error("Bad key/value separator {src[pos]}")
			pos += 1
			ignore_whitespaces
			var value = parse_entity
			#print "Parsed value {value} for JSON object"
			if value isa JsonParseError then return value
			obj[key] = value
			ignore_whitespaces
			if pos >= max then return make_parse_error("Incomplete JSON object")
			c = src[pos]
			if c == '}' then break
			if c != ',' then return make_parse_error("Bad object separator {src[pos]}")
			pos += 1
		end
		pos += 1
		return obj
	end

	# Creates a `JsonParseError` with the right message and location
	protected fun make_parse_error(message: String): JsonParseError do
		var err = new JsonParseError(message)
		err.location = hot_location
		return err
	end

	# Parses an Int or Float
	fun parse_json_number: Serializable do
		var max = len
		var p = pos
		var c = src[p]
		var is_neg = false
		if c == '-' then
			is_neg = true
			p += 1
			if p >= max then return make_parse_error("Bad JSON number")
			c = src[p]
		end
		var val = 0
		while c.is_numeric do
			val *= 10
			val += c.to_i
			p += 1
			if p >= max then break
			c = src[p]
		end
		if c == '.' then
			p += 1
			if p >= max then return make_parse_error("Bad JSON number")
			c = src[p]
			var fl = val.to_f
			var frac = 0.1
			while c.is_numeric do
				fl += c.to_i.to_f * frac
				frac /= 10.0
				p += 1
				if p >= max then break
				c = src[p]
			end
			if c == 'e' or c == 'E' then
				p += 1
				var exp = 0
				if p >= max then return make_parse_error("Malformed JSON number")
				c = src[p]
				while c.is_numeric do
					exp *= 10
					exp += c.to_i
					p += 1
					if p >= max then break
					c = src[p]
				end
				fl *= (10 ** exp).to_f
			end
			if p < max and not c.is_json_separator then return make_parse_error("Malformed JSON number")
			pos = p
			if is_neg then return -fl
			return fl
		end
		if c == 'e' or c == 'E' then
			p += 1
			if p >= max then return make_parse_error("Bad JSON number")
			var exp = src[p].to_i
			c = src[p]
			while c.is_numeric do
				exp *= 10
				exp += c.to_i
				p += 1
				if p >= max then break
				c = src[p]
			end
			val *= (10 ** exp)
		end
		if p < max and not src[p].is_json_separator then return make_parse_error("Malformed JSON number")
		pos = p
		if is_neg then return -val
		return val
	end

	private var parse_str_buf = new FlatBuffer

	# Parses and returns a Nit string from a JSON String
	fun parse_json_string: Serializable do
		var src = src
		var ln = src.length
		var p = pos
		p += 1
		if p > ln then return make_parse_error("Malformed JSON String")
		var c = src[p]
		var ret = parse_str_buf
		var chunk_st = p
		while c != '"' do
			if c != '\\' then
				p += 1
				if p >= ln then return make_parse_error("Malformed JSON string")
				c = src[p]
				continue
			end
			ret.append_substring_impl(src, chunk_st, p - chunk_st)
			p += 1
			if p >= ln then return make_parse_error("Malformed Escape sequence in JSON string")
			c = src[p]
			if c == 'r' then
				ret.add '\r'
				p += 1
			else if c == 'n' then
				ret.add '\n'
				p += 1
			else if c == 't' then
				ret.add '\t'
				p += 1
			else if c == 'u' then
				var cp = 0
				p += 1
				for i in [0 .. 4[ do
					cp <<= 4
					if p >= ln then make_parse_error("Malformed \uXXXX Escape sequence in JSON string")
					c = src[p]
					if c >= '0' and c <= '9' then
						cp += c.code_point - '0'.code_point
					else if c >= 'a' and c <= 'f' then
						cp += c.code_point - 'a'.code_point + 10
					else if c >= 'A' and c <= 'F' then
						cp += c.code_point - 'A'.code_point + 10
					else
						make_parse_error("Malformed \uXXXX Escape sequence in JSON string")
					end
					p += 1
				end
				c = cp.code_point
				if cp >= 0xD800 and cp <= 0xDBFF then
					if p >= ln then make_parse_error("Malformed \uXXXX Escape sequence in JSON string")
					c = src[p]
					if c != '\\' then make_parse_error("Malformed \uXXXX Escape sequence in JSON string")
					p += 1
					c = src[p]
					if c != 'u' then make_parse_error("Malformed \uXXXX Escape sequence in JSON string")
					var locp = 0
					p += 1
					for i in [0 .. 4[ do
						locp <<= 4
						if p > ln then make_parse_error("Malformed \uXXXX Escape sequence in JSON string")
						c = src[p]
						if c >= '0' and c <= '9' then
							locp += c.code_point - '0'.code_point
						else if c >= 'a' and c <= 'f' then
							locp += c.code_point - 'a'.code_point + 10
						else if c >= 'A' and c <= 'F' then
							locp += c.code_point - 'A'.code_point + 10
						else
							make_parse_error("Malformed \uXXXX Escape sequence in JSON string")
						end
						p += 1
					end
					c = (((locp & 0x3FF) | ((cp & 0x3FF) << 10)) + 0x10000).code_point
				end
				ret.add c
			else if c == 'b' then
				ret.add 8.code_point
				p += 1
			else if c == 'f' then
				ret.add '\f'
				p += 1
			else
				p += 1
				ret.add c
			end
			chunk_st = p
			c = src[p]
		end
		pos = p + 1
		if ret.is_empty then return src.substring(chunk_st, p - chunk_st)
		ret.append_substring_impl(src, chunk_st, p - chunk_st)
		var rets = ret.to_s
		ret.clear
		return rets
	end

	# Ignores any character until a JSON separator is encountered
	fun ignore_until_separator do
		var max = len
		while pos < max do
			if not src[pos].is_json_separator then return
		end
	end
end