fediglam/src/template/lib.zig

const std = @import("std");

pub fn main() !void {
    try execute(std.io.getStdOut().writer(), @embedFile("./test.tmp.html"), .{ .community = .{ .name = "abcd" } });
}

const logging = false;

pub fn execute(writer: anytype, comptime template: []const u8, args: anytype) !void {
    const items = comptime parseTemplate(template);
    try executeTemplate(writer, items, args);
}

fn executeTemplate(writer: anytype, comptime items: []const TemplateItem, args: anytype) !void {
    inline for (items) |it| switch (it) {
        .text => |text| try writer.writeAll(text),
        .statement => |stmt| try executeStatement(writer, stmt, args),
    };
}

fn executeStatement(writer: anytype, comptime stmt: Statement, args: anytype) !void {
    switch (stmt) {
        .expression => |expr| switch (expr) {
            .arg_deref => |fields| try argDeref(writer, fields, args),
        },
        else => @compileError("TODO"),
    }
}

fn argDeref(writer: anytype, comptime names: []const []const u8, arg: anytype) !void {
    if (names.len == 0) {
        const T = @TypeOf(arg);
        if (comptime std.meta.trait.isZigString(T)) return writer.writeAll(arg);
        return std.fmt.format(writer, "{any}", .{arg});
    }

    return argDeref(writer, names[1..], @field(arg, names[0]));
}

fn parseTemplate(comptime template: []const u8) []const TemplateItem {
    comptime {
        @setEvalBranchQuota(@intCast(u32, template.len * 6));
        var iter = TokenIter{ .text = template };
        var items: []const TemplateItem = &.{};
        var current_text: []const u8 = "";

        while (iter.next()) |token| {
            if (logging) @compileLog(token);
            switch (token) {
                .whitespace, .text => |text| current_text = current_text ++ text,
                .open_bracket => {
                    const next = iter.peek() orelse @compileError("Unexpected end of template");
                    if (next == .open_bracket) {
                        current_text = current_text ++ "{";
                        _ = iter.next();
                    } else {
                        if (current_text.len != 0) {
                            items = items ++ [_]TemplateItem{.{ .text = current_text }};
                            current_text = "";
                        }
                        const result = parseStatement(iter);
                        iter = result.new_iter;
                        items = items ++ [_]TemplateItem{.{ .statement = result.item }};
                    }
                },
                .close_bracket => {
                    const next = iter.next() orelse @compileError("Unexpected end of template");
                    if (next == .close_bracket) current_text = current_text ++ "}" else @compileError("Unpaired close bracket, did you mean \"}}\"?");
                },
                .period => current_text = current_text ++ ".",
                .pound => current_text = current_text ++ "#",
            }
        }

        if (current_text.len != 0) {
            items = items ++ [_]TemplateItem{.{ .text = current_text }};
        }

        return items;
    }
}

fn parseStatement(comptime tokens: TokenIter) ParseResult(Statement) {
    comptime {
        var iter = tokens;
        while (iter.next()) |token| switch (token) {
            .whitespace => {},
            .pound => {
                const next = iter.next() orelse @compileError("Unexpected end of template");
                if (logging) @compileLog("keyword", next);
                if (next != .text) @compileError("Expected keyword following '#' character");
                const text = next.text;
                const keyword = std.meta.stringToEnum(Keyword, text) orelse @compileError("Unknown keyword: " ++ text);

                _ = keyword;
                @panic("todo");
            },
            .period => {
                const expr = parseArgDeref(iter);
                iter = expr.new_iter;
                while (iter.next()) |it| switch (it) {
                    .whitespace => {},
                    .close_bracket => break,
                    else => @compileError("TODO"),
                };
                return .{ .new_iter = iter, .item = .{
                    .expression = .{ .arg_deref = expr.item },
                } };
            },
            else => @compileError(""),
        };

        @compileError("Unexpected end of template");
    }
}

fn parseArgDeref(comptime tokens: TokenIter) ParseResult([]const []const u8) {
    comptime {
        var iter = tokens;
        var fields: []const []const u8 = &.{};
        var wants = .text;
        while (iter.peek()) |token| {
            switch (token) {
                .whitespace => {},
                .text => |text| {
                    if (wants != .text) @compileError("Unexpected token \"" ++ text ++ "\"");
                    fields = fields ++ [1][]const u8{text};
                    wants = .period;
                },
                .period => {
                    if (wants != .period) @compileError("Unexpected token \".\"");
                    wants = .text;
                },
                else => if (wants == .period) return .{
                    .new_iter = iter,
                    .item = fields,
                } else @compileError("Unexpected token"),
            }
            _ = iter.next();
        }
    }
}

fn ParseResult(comptime T: type) type {
    return struct {
        new_iter: TokenIter,
        item: T,
    };
}

const TemplateItem = union(enum) {
    text: []const u8,
    statement: Statement,
};

const Expression = union(enum) {
    arg_deref: []const []const u8,
};

const Statement = union(enum) {
    expression: Expression,
    for_loop: struct {
        subtemplate: []const u8,
        indexable: Expression,
        iteration_capture: []const u8,
        index_capture: ?[]const u8,
    },
};

const State = enum {
    text,
    text_close_bracket,
    template_start,
    template,
};

const Keyword = enum {
    @"for",
};

const Token = union(enum) {
    text: []const u8,
    open_bracket: void,
    close_bracket: void,
    period: void,
    whitespace: []const u8,
    pound: void,
};

const TokenIter = struct {
    start: usize = 0,
    text: []const u8,
    peeked_token: ?Token = null,

    fn next(self: *TokenIter) ?Token {
        if (self.peeked_token) |token| {
            self.peeked_token = null;
            return token;
        }

        const remaining = self.text[self.start..];
        if (remaining.len == 0) return null;

        const ch = remaining[0];
        self.start += 1;
        switch (ch) {
            '{' => return .{ .open_bracket = {} },
            '}' => return .{ .close_bracket = {} },
            '.' => return .{ .period = {} },
            '#' => return .{ .pound = {} },
            ' ', '\t', '\n', '\r' => {
                var idx: usize = 0;
                while (idx < remaining.len and std.mem.indexOfScalar(u8, " \t\n\r", remaining[idx]) != null) : (idx += 1) {}

                self.start += idx - 1;
                return .{ .whitespace = remaining[0..idx] };
            },
            else => {
                var idx: usize = 0;
                while (idx < remaining.len and std.mem.indexOfScalar(u8, "{}.# \t\n\r", remaining[idx]) == null) : (idx += 1) {}

                self.start += idx - 1;
                return .{ .text = remaining[0..idx] };
            },
        }
    }

    fn peek(self: *TokenIter) ?Token {
        const token = self.next();
        self.peeked_token = token;
        return token;
    }
};