/// Build-time generator: reads ie_data.csv and outputs a Zig source file
/// containing the parsed Shiller annual returns as a const array.
///
/// Uses the same algorithm as the former comptime parser:
///   - Skip 8 header lines
///   - Scan each line, fast-reject non-January by checking "YYYY.01," at fixed offsets
///   - Parse cols 4 (CPI), 6 (GS10), 9 (TR Price) via CsvFieldIterator
///   - Compute year-over-year returns on the fly
///   - Skip ~11 months after each January row
const std = @import("std");
const ShillerYear = @import("shiller").ShillerYear;

pub fn main() !void {
    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
    defer arena.deinit();
    const allocator = arena.allocator();

    const args = try std.process.argsAlloc(allocator);
    if (args.len < 3) {
        std.debug.print("Usage: gen_shiller <ie_data.csv> <output.zig>\n", .{});
        std.process.exit(1);
    }

    const csv_data = try std.fs.cwd().readFileAlloc(allocator, args[1], 10 * 1024 * 1024);

    var results: [200]ShillerYear = undefined;

    // Write output .zig file — just raw parallel arrays, no type dependencies.
    const out_file = try std.fs.cwd().createFile(args[2], .{});
    defer out_file.close();

    const parsed = try parseCsv(csv_data, &results);
    var out_buf: [1024]u8 = undefined;
    var file_writer = out_file.writer(&out_buf);
    const writer = &file_writer.interface;
    try writer.writeAll(
        \\// Auto-generated from ie_data.csv — do not edit.
        \\// Regenerate: zig build (runs build/gen_shiller.zig)
        \\
        \\const ShillerYear = @import("shiller").ShillerYear;
        \\
        \\pub const data = [_]ShillerYear{
        \\
    );

    for (parsed) |p|
        try writer.print(
            "    .{{ .year = {d}, .sp500_total_return = {d}, .bond_total_return = {d}, .cpi_inflation = {d} }},\n",
            .{ p.year, p.sp500_total_return, p.bond_total_return, p.cpi_inflation },
        );

    try writer.writeAll("};\n");
    try writer.flush();
}

fn parseCsv(csv_data: []const u8, buffer: []ShillerYear) ![]ShillerYear {
    var prev_year: u16 = 0;
    var prev_tr_price: f64 = 0;
    var prev_gs10: f64 = 0;
    var prev_cpi: f64 = 0;

    // Skip 8 header lines
    var pos: usize = 0;
    var newlines: usize = 0;
    while (pos < csv_data.len and newlines < 8) : (pos += 1) {
        if (csv_data[pos] == '\n') newlines += 1;
    }

    const skip_bytes: usize = 11 * 96;

    var current: usize = 0;
    while (pos < csv_data.len) {
        const line_start = pos;
        while (pos < csv_data.len and csv_data[pos] != '\n') pos += 1;
        const line_end = if (pos > line_start and csv_data[pos - 1] == '\r') pos - 1 else pos;
        if (pos < csv_data.len) pos += 1;

        const line = csv_data[line_start..line_end];
        if (line.len < 7) continue;

        // Fast reject: date is "YYYY.01,"
        if (line[4] != '.' or line[5] != '0' or line[6] != '1') continue;

        const year = std.fmt.parseInt(u16, line[0..4], 10) catch continue;

        // Parse fields via CsvFieldIterator
        var col_iter = CsvFieldIterator{ .data = line };
        _ = col_iter.next(); // col 0: Date
        _ = col_iter.next(); // col 1: P
        _ = col_iter.next(); // col 2: D
        _ = col_iter.next(); // col 3: E
        const cpi_field = col_iter.next() orelse continue;
        _ = col_iter.next(); // col 5
        const gs10_field = col_iter.next() orelse continue;
        _ = col_iter.next(); // col 7
        _ = col_iter.next(); // col 8
        const tr_price_field = col_iter.next() orelse continue;

        const cpi = parseF64WithCommas(cpi_field) orelse continue;
        const gs10 = parseF64WithCommas(gs10_field) orelse continue;
        const tr_price = parseF64WithCommas(tr_price_field) orelse continue;

        if (cpi == 0.0 or tr_price == 0.0 or gs10 == 0.0) continue;

        if (prev_year > 0 and year == prev_year + 1) {
            const cpi_change = (cpi / prev_cpi) - 1.0;
            const real_sp500 = (tr_price / prev_tr_price) - 1.0;

            buffer[current] = .{
                .year = prev_year,
                .sp500_total_return = (1.0 + real_sp500) * (1.0 + cpi_change) - 1.0,
                .bond_total_return = prev_gs10 / 100.0,
                .cpi_inflation = cpi_change,
            };
            current += 1;
        }

        prev_year = year;
        prev_tr_price = tr_price;
        prev_gs10 = gs10;
        prev_cpi = cpi;

        // Skip ~11 months
        pos = @min(pos + skip_bytes, csv_data.len);
        while (pos < csv_data.len and csv_data[pos] != '\n') pos += 1;
        if (pos < csv_data.len) pos += 1;
    }
    return buffer[0..current];
}
// --- Parsing helpers (same as shiller.zig's former comptime versions) ---

const CsvFieldIterator = struct {
    data: []const u8,
    pos: usize = 0,

    fn next(self: *CsvFieldIterator) ?[]const u8 {
        if (self.pos > self.data.len) return null;
        if (self.pos == self.data.len) {
            self.pos = self.data.len + 1;
            return "";
        }

        const start = self.pos;

        // Handle quoted fields
        if (self.pos < self.data.len and self.data[self.pos] == '"') {
            self.pos += 1;
            const qstart = self.pos;
            while (self.pos < self.data.len) {
                if (self.data[self.pos] == '"') {
                    if (self.pos + 1 < self.data.len and self.data[self.pos + 1] == '"') {
                        self.pos += 2;
                    } else {
                        break;
                    }
                } else {
                    self.pos += 1;
                }
            }
            const qend = self.pos;
            if (self.pos < self.data.len) self.pos += 1;
            if (self.pos < self.data.len and self.data[self.pos] == ',') self.pos += 1;
            return self.data[qstart..qend];
        }

        // Unquoted field
        while (self.pos < self.data.len and self.data[self.pos] != ',') {
            self.pos += 1;
        }
        const end = self.pos;
        if (self.pos < self.data.len) self.pos += 1;
        return self.data[start..end];
    }
};

fn parseF64WithCommas(s: []const u8) ?f64 {
    if (s.len == 0) return null;

    var start: usize = 0;
    var end: usize = s.len;
    while (start < end and s[start] == ' ') start += 1;
    while (end > start and s[end - 1] == ' ') end -= 1;
    if (start >= end) return null;

    var negative = false;
    if (s[start] == '-') {
        negative = true;
        start += 1;
    }

    var integer_part: f64 = 0;
    var i = start;
    while (i < end and s[i] != '.') : (i += 1) {
        if (s[i] == ',') continue;
        if (s[i] < '0' or s[i] > '9') return null;
        integer_part = integer_part * 10.0 + @as(f64, @floatFromInt(s[i] - '0'));
    }

    var frac_part: f64 = 0;
    if (i < end and s[i] == '.') {
        i += 1;
        var divisor: f64 = 10.0;
        while (i < end) : (i += 1) {
            if (s[i] < '0' or s[i] > '9') return null;
            frac_part += @as(f64, @floatFromInt(s[i] - '0')) / divisor;
            divisor *= 10.0;
        }
    }

    const result = integer_part + frac_part;
    return if (negative) -result else result;
}