zig/lib/std/json/write_stream.zig

const std = @import("../std.zig");
const assert = std.debug.assert;
const maxInt = std.math.maxInt;

const State = enum {
    Complete,
    Value,
    ArrayStart,
    Array,
    ObjectStart,
    Object,
};

/// Writes JSON ([RFC8259](https://tools.ietf.org/html/rfc8259)) formatted data
/// to a stream. `max_depth` is a comptime-known upper bound on the nesting depth.
/// TODO A future iteration of this API will allow passing `null` for this value,
/// and disable safety checks in release builds.
pub fn WriteStream(comptime OutStream: type, comptime max_depth: usize) type {
    return struct {
        const Self = @This();

        pub const Stream = OutStream;

        /// The string used for indenting.
        one_indent: []const u8 = " ",

        /// The string used as a newline character.
        newline: []const u8 = "\n",

        /// The string used as spacing.
        space: []const u8 = " ",

        stream: *OutStream,
        state_index: usize,
        state: [max_depth]State,

        pub fn init(stream: *OutStream) Self {
            var self = Self{
                .stream = stream,
                .state_index = 1,
                .state = undefined,
            };
            self.state[0] = .Complete;
            self.state[1] = .Value;
            return self;
        }

        pub fn beginArray(self: *Self) !void {
            assert(self.state[self.state_index] == State.Value); // need to call arrayElem or objectField
            try self.stream.writeByte('[');
            self.state[self.state_index] = State.ArrayStart;
        }

        pub fn beginObject(self: *Self) !void {
            assert(self.state[self.state_index] == State.Value); // need to call arrayElem or objectField
            try self.stream.writeByte('{');
            self.state[self.state_index] = State.ObjectStart;
        }

        pub fn arrayElem(self: *Self) !void {
            const state = self.state[self.state_index];
            switch (state) {
                .Complete => unreachable,
                .Value => unreachable,
                .ObjectStart => unreachable,
                .Object => unreachable,
                .Array, .ArrayStart => {
                    if (state == .Array) {
                        try self.stream.writeByte(',');
                    }
                    self.state[self.state_index] = .Array;
                    self.pushState(.Value);
                    try self.indent();
                },
            }
        }

        pub fn objectField(self: *Self, name: []const u8) !void {
            const state = self.state[self.state_index];
            switch (state) {
                .Complete => unreachable,
                .Value => unreachable,
                .ArrayStart => unreachable,
                .Array => unreachable,
                .Object, .ObjectStart => {
                    if (state == .Object) {
                        try self.stream.writeByte(',');
                    }
                    self.state[self.state_index] = .Object;
                    self.pushState(.Value);
                    try self.indent();
                    try self.writeEscapedString(name);
                    try self.stream.write(":");
                    try self.stream.write(self.space);
                },
            }
        }

        pub fn endArray(self: *Self) !void {
            switch (self.state[self.state_index]) {
                .Complete => unreachable,
                .Value => unreachable,
                .ObjectStart => unreachable,
                .Object => unreachable,
                .ArrayStart => {
                    try self.stream.writeByte(']');
                    self.popState();
                },
                .Array => {
                    try self.indent();
                    self.popState();
                    try self.stream.writeByte(']');
                },
            }
        }

        pub fn endObject(self: *Self) !void {
            switch (self.state[self.state_index]) {
                .Complete => unreachable,
                .Value => unreachable,
                .ArrayStart => unreachable,
                .Array => unreachable,
                .ObjectStart => {
                    try self.stream.writeByte('}');
                    self.popState();
                },
                .Object => {
                    try self.indent();
                    self.popState();
                    try self.stream.writeByte('}');
                },
            }
        }

        pub fn emitNull(self: *Self) !void {
            assert(self.state[self.state_index] == State.Value);
            try self.stream.write("null");
            self.popState();
        }

        pub fn emitBool(self: *Self, value: bool) !void {
            assert(self.state[self.state_index] == State.Value);
            if (value) {
                try self.stream.write("true");
            } else {
                try self.stream.write("false");
            }
            self.popState();
        }

        pub fn emitNumber(
            self: *Self,
            /// An integer, float, or `std.math.BigInt`. Emitted as a bare number if it fits losslessly
            /// in a IEEE 754 double float, otherwise emitted as a string to the full precision.
            value: var,
        ) !void {
            assert(self.state[self.state_index] == State.Value);
            switch (@typeInfo(@TypeOf(value))) {
                .Int => |info| {
                    if (info.bits < 53) {
                        try self.stream.print("{}", .{value});
                        self.popState();
                        return;
                    }
                    if (value < 4503599627370496 and (!info.is_signed or value > -4503599627370496)) {
                        try self.stream.print("{}", .{value});
                        self.popState();
                        return;
                    }
                },
                .Float => if (@floatCast(f64, value) == value) {
                    try self.stream.print("{}", .{value});
                    self.popState();
                    return;
                },
                else => {},
            }
            try self.stream.print("\"{}\"", .{value});
            self.popState();
        }

        pub fn emitString(self: *Self, string: []const u8) !void {
            try self.writeEscapedString(string);
            self.popState();
        }

        fn writeEscapedString(self: *Self, string: []const u8) !void {
            try self.stream.writeByte('"');
            for (string) |s| {
                switch (s) {
                    '"' => try self.stream.write("\\\""),
                    '\t' => try self.stream.write("\\t"),
                    '\r' => try self.stream.write("\\r"),
                    '\n' => try self.stream.write("\\n"),
                    8 => try self.stream.write("\\b"),
                    12 => try self.stream.write("\\f"),
                    '\\' => try self.stream.write("\\\\"),
                    else => try self.stream.writeByte(s),
                }
            }
            try self.stream.writeByte('"');
        }

        /// Writes the complete json into the output stream
        pub fn emitJson(self: *Self, json: std.json.Value) Stream.Error!void {
            switch (json) {
                .Null => try self.emitNull(),
                .Bool => |inner| try self.emitBool(inner),
                .Integer => |inner| try self.emitNumber(inner),
                .Float => |inner| try self.emitNumber(inner),
                .String => |inner| try self.emitString(inner),
                .Array => |inner| {
                    try self.beginArray();
                    for (inner.toSliceConst()) |elem| {
                        try self.arrayElem();
                        try self.emitJson(elem);
                    }
                    try self.endArray();
                },
                .Object => |inner| {
                    try self.beginObject();
                    var it = inner.iterator();
                    while (it.next()) |entry| {
                        try self.objectField(entry.key);
                        try self.emitJson(entry.value);
                    }
                    try self.endObject();
                },
            }
        }

        fn indent(self: *Self) !void {
            assert(self.state_index >= 1);
            try self.stream.write(self.newline);
            var i: usize = 0;
            while (i < self.state_index - 1) : (i += 1) {
                try self.stream.write(self.one_indent);
            }
        }

        fn pushState(self: *Self, state: State) void {
            self.state_index += 1;
            self.state[self.state_index] = state;
        }

        fn popState(self: *Self) void {
            self.state_index -= 1;
        }
    };
}

test "json write stream" {
    var out_buf: [1024]u8 = undefined;
    var slice_stream = std.io.SliceOutStream.init(&out_buf);
    const out = &slice_stream.stream;

    var arena_allocator = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer arena_allocator.deinit();

    var w = std.json.WriteStream(@TypeOf(out).Child, 10).init(out);
    try w.emitJson(try getJson(&arena_allocator.allocator));

    const result = slice_stream.getWritten();
    const expected =
        \\{
        \\ "object": {
        \\  "one": 1,
        \\  "two": 2.0e+00
        \\ },
        \\ "string": "This is a string",
        \\ "array": [
        \\  "Another string",
        \\  1,
        \\  3.14e+00
        \\ ],
        \\ "int": 10,
        \\ "float": 3.14e+00
        \\}
    ;
    std.testing.expect(std.mem.eql(u8, expected, result));
}

fn getJson(allocator: *std.mem.Allocator) !std.json.Value {
    var value = std.json.Value{ .Object = std.json.ObjectMap.init(allocator) };
    _ = try value.Object.put("string", std.json.Value{ .String = "This is a string" });
    _ = try value.Object.put("int", std.json.Value{ .Integer = @intCast(i64, 10) });
    _ = try value.Object.put("float", std.json.Value{ .Float = 3.14 });
    _ = try value.Object.put("array", try getJsonArray(allocator));
    _ = try value.Object.put("object", try getJsonObject(allocator));
    return value;
}

fn getJsonObject(allocator: *std.mem.Allocator) !std.json.Value {
    var value = std.json.Value{ .Object = std.json.ObjectMap.init(allocator) };
    _ = try value.Object.put("one", std.json.Value{ .Integer = @intCast(i64, 1) });
    _ = try value.Object.put("two", std.json.Value{ .Float = 2.0 });
    return value;
}

fn getJsonArray(allocator: *std.mem.Allocator) !std.json.Value {
    var value = std.json.Value{ .Array = std.json.Array.init(allocator) };
    var array = &value.Array;
    _ = try array.append(std.json.Value{ .String = "Another string" });
    _ = try array.append(std.json.Value{ .Integer = @intCast(i64, 1) });
    _ = try array.append(std.json.Value{ .Float = 3.14 });

    return value;
}