path: root/src/sam_edit.zig

//! acme's Edit (editors/acme/edit.c, ecmd.c, elog.c): sam's command
//! language over one pane's body. The commands are parsed as edit.c parses
//! them, run as ecmd.c runs them against the text as it was before the Edit,
//! and their changes collected in order, as elog.c logs them, then applied
//! at once: an Edit is one undo step, and one that fails changes nothing.
//!
//! Addresses are the `addr` file's (src/ninep/addr.zig), so they take
//! pardes's `line:col` too; regular expressions are src/regexp.zig's, mvzr
//! searched as sam searches, with its step budget.
//!
//! Left out: the file commands (`b B D e r w f X Y`), the pipes (`< | >`),
//! the `'` and `"` addresses, and `\1`..`\9` in `s` (mvzr has no
//! submatches). `u` undoes or redoes only as a whole Edit. acme warns about
//! changes out of sequence (elog.c:130) and applies them anyway; here the
//! Edit is refused.
const std = @import("std");
const regexp = @import("regexp.zig");
const addr_lang = @import("ninep/addr.zig");

pub const Range = addr_lang.Range;

pub const Op = struct { q0: u32, q1: u32, text: []const u8 };

/// What an Edit came to: the changes in order, the dot it leaves (in the
/// text before them), and what `p` and `=` printed.
pub const Result = struct {
    ops: []const Op,
    dot: Range,
    printed: []const u8,
    /// A bare `u`/`u-3`: undo (positive) or redo (negative) that many.
    undo: i32 = 0,
};

pub const Failure = error{ Edit, OutOfMemory };

/// Why an Edit failed, in acme's words, when `run` answered error.Edit.
pub const Why = struct {
    buf: [200]u8 = undefined,
    len: usize = 0,
    pub fn text(w: *const Why) []const u8 {
        return w.buf[0..w.len];
    }
};

const Cmd = struct {
    c: u8,
    addr: ?[]const u8 = null,
    re: ?[]const u8 = null,
    text: []const u8 = "",
    sub: ?*Cmd = null,
    next: ?*Cmd = null,
    num: i32 = 0,
    global: bool = false,
    to: ?[]const u8 = null,
};

/// Runs `command` over `text` from `dot`, the file named `name` (for `=`).
/// Everything returned lives in `arena`.
pub fn run(arena: std.mem.Allocator, text: []const u8, dot: Range, name: []const u8, command: []const u8, why: *Why) Failure!Result {
    var src = try arena.alloc(u8, command.len + 1);
    @memcpy(src[0..command.len], command);
    src[command.len] = '\n';
    if (command.len > 0 and command[command.len - 1] == '\n') src = src[0..command.len];
    var ps: Parser = .{ .arena = arena, .src = src, .why = why };
    var first: ?*Cmd = null;
    var last: ?*Cmd = null;
    while (try ps.parse(0)) |c| {
        if (last) |l| l.next = c else first = c;
        last = c;
    }
    const cmds = first orelse return .{ .ops = &.{}, .dot = dot, .printed = "" };
    // acme's u runs at once (ecmd.c:532), outside the edit log; here it is
    // an Edit of its own.
    if (cmds.c == 'u') {
        if (cmds.next != null) return fail(why, "u is only an Edit of its own here", .{});
        return .{ .ops = &.{}, .dot = dot, .printed = "", .undo = cmds.num };
    }
    var ex: Exec = .{ .arena = arena, .text = text, .name = name, .why = why };
    var d = dot;
    var it: ?*Cmd = cmds;
    while (it) |c| : (it = c.next) d = try ex.exec(c, d);
    return .{ .ops = ex.ops.items, .dot = d, .printed = ex.out.items };
}

fn fail(why: *Why, comptime fmt: []const u8, args: anytype) Failure {
    why.len = if (std.fmt.bufPrint(&why.buf, fmt, args)) |s| s.len else |_| 0;
    return error.Edit;
}

// ---- parsing: edit.c ----

const Parser = struct {
    arena: std.mem.Allocator,
    src: []const u8,
    i: usize = 0,
    /// The last pattern given, for an empty one (edit.c:562, getregexp).
    lastpat: []const u8 = "",
    why: *Why,

    fn getch(ps: *Parser) ?u8 {
        if (ps.i >= ps.src.len) return null;
        ps.i += 1;
        return ps.src[ps.i - 1];
    }

    fn nextc(ps: *const Parser) ?u8 {
        return if (ps.i < ps.src.len) ps.src[ps.i] else null;
    }

    fn skipbl(ps: *Parser) ?u8 {
        while (ps.nextc()) |c| {
            if (c != ' ' and c != '\t') return c;
            ps.i += 1;
        }
        return null;
    }

    fn atnl(ps: *Parser) Failure!void {
        _ = ps.skipbl();
        const c = ps.getch() orelse '\n';
        if (c != '\n') return fail(ps.why, "newline expected (saw {c})", .{c});
    }

    fn okdelim(ps: *Parser, c: u8) Failure!void {
        if (c == '\\' or std.ascii.isAlphanumeric(c)) return fail(ps.why, "bad delimiter {c}", .{c});
    }

    /// edit.c:216: a count, 1 when there is none; `-` allowed when signok > 1.
    fn getnum(ps: *Parser, signok: u8) i32 {
        var sign: i32 = 1;
        if (signok > 1 and ps.nextc() == '-') {
            sign = -1;
            ps.i += 1;
        }
        const c = ps.nextc() orelse return sign;
        if (c < '0' or c > '9') return sign;
        var n: i32 = 0;
        while (ps.nextc()) |d| {
            if (d < '0' or d > '9') break;
            n = n *| 10 +| (d - '0');
            ps.i += 1;
        }
        return sign * n;
    }

    /// An address's text, blanks dropped, or null: the syntax of edit.c's
    /// compoundaddr and simpleaddr, evaluated later by addr.zig.
    fn address(ps: *Parser) Failure!?[]const u8 {
        var out: std.ArrayList(u8) = .empty;
        while (true) {
            const c = ps.skipbl() orelse break;
            switch (c) {
                '0'...'9', '#', '.', '$', '+', '-', ',', ';' => {
                    ps.i += 1;
                    try out.append(ps.arena, c);
                    // pardes's line:col
                    if (c >= '0' and c <= '9' and ps.nextc() == ':' and ps.i + 1 < ps.src.len and
                        ps.src[ps.i + 1] >= '0' and ps.src[ps.i + 1] <= '9')
                    {
                        ps.i += 1;
                        try out.append(ps.arena, ':');
                    }
                },
                '/', '?' => {
                    ps.i += 1;
                    try out.append(ps.arena, c);
                    while (ps.nextc()) |d| {
                        if (d == '\n') break;
                        ps.i += 1;
                        try out.append(ps.arena, d);
                        if (d == '\\') {
                            if (ps.nextc()) |e| if (e != '\n') {
                                ps.i += 1;
                                try out.append(ps.arena, e);
                            };
                            continue;
                        }
                        if (d == c) break;
                    }
                },
                '\'' => return fail(ps.why, "can't handle '", .{}),
                '"' => return fail(ps.why, "file addresses are not supported", .{}),
                else => break,
            }
        }
        return if (out.items.len == 0) null else out.items;
    }

    /// edit.c:562: up to the delimiter; `\delim` is the delimiter, `\\` stays.
    fn getregexp(ps: *Parser, delim: u8) Failure![]const u8 {
        var buf: std.ArrayList(u8) = .empty;
        while (true) {
            var c = ps.getch() orelse break;
            if (c == '\\') {
                if (ps.nextc() == delim) {
                    c = ps.getch().?;
                } else if (ps.nextc() == '\\') {
                    try buf.append(ps.arena, c);
                    c = ps.getch().?;
                }
            } else if (c == delim or c == '\n') {
                if (c == '\n') ps.i -= 1;
                break;
            }
            try buf.append(ps.arena, c);
        }
        if (buf.items.len > 0) ps.lastpat = buf.items;
        if (ps.lastpat.len == 0) return fail(ps.why, "no regular expression defined", .{});
        return ps.lastpat;
    }

    /// edit.c:388: `\n` is a newline; for `s` every other escape is kept
    /// for it to read (`\&`, `\\`).
    fn getrhs(ps: *Parser, delim: u8, cmd: u8) Failure![]const u8 {
        var s: std.ArrayList(u8) = .empty;
        while (ps.getch()) |c0| {
            var c = c0;
            if (c == delim or c == '\n') {
                ps.i -= 1;
                break;
            }
            if (c == '\\') {
                c = ps.getch() orelse return fail(ps.why, "bad right hand side", .{});
                if (c == '\n') {
                    ps.i -= 1;
                    c = '\\';
                } else if (c == 'n') {
                    c = '\n';
                } else if (c != delim and (cmd == 's' or c != '\\')) {
                    try s.append(ps.arena, '\\');
                }
            }
            try s.append(ps.arena, c);
        }
        return s.items;
    }

    /// edit.c:425: `/text/` on the line, or lines up to one that is `.`.
    fn collecttext(ps: *Parser) Failure![]const u8 {
        if (ps.skipbl() == '\n') {
            ps.i += 1;
            var s: std.ArrayList(u8) = .empty;
            while (ps.i < ps.src.len) {
                const end = std.mem.indexOfScalarPos(u8, ps.src, ps.i, '\n') orelse ps.src.len;
                const line = ps.src[ps.i..end];
                ps.i = @min(end + 1, ps.src.len);
                if (std.mem.eql(u8, line, ".")) return s.items;
                try s.appendSlice(ps.arena, line);
                try s.append(ps.arena, '\n');
            }
            return s.items;
        }
        const delim = ps.getch() orelse return "";
        try ps.okdelim(delim);
        const s = try ps.getrhs(delim, 'a');
        if (ps.nextc() == delim) ps.i += 1;
        try ps.atnl();
        return s;
    }

    /// edit.c:467, parsecmd.
    fn parse(ps: *Parser, nest: u32) Failure!?*Cmd {
        const a = try ps.address();
        _ = ps.skipbl() orelse return null;
        const c = ps.getch() orelse return null;
        const cmd = try ps.arena.create(Cmd);
        cmd.* = .{ .c = c, .addr = a };
        switch (c) {
            '\n' => {},
            'a', 'c', 'i' => cmd.text = try ps.collecttext(),
            'd', 'p' => try ps.atnl(),
            '=' => {
                _ = ps.skipbl();
                const start = ps.i;
                while (ps.nextc()) |d| : (ps.i += 1) if (d == '\n') break;
                cmd.text = std.mem.trimEnd(u8, ps.src[start..ps.i], " \t");
                ps.i = @min(ps.i + 1, ps.src.len);
                if (cmd.text.len != 0 and !std.mem.eql(u8, cmd.text, "#")) return fail(ps.why, "newline expected", .{});
            },
            's' => {
                cmd.num = ps.getnum(1);
                _ = ps.skipbl();
                const delim = ps.getch() orelse '\n';
                if (delim == '\n') return fail(ps.why, "no address", .{});
                try ps.okdelim(delim);
                cmd.re = try ps.getregexp(delim);
                cmd.text = try ps.getrhs(delim, 's');
                if (ps.nextc() == delim) {
                    ps.i += 1;
                    if (ps.nextc() == 'g') {
                        ps.i += 1;
                        cmd.global = true;
                    }
                }
                try ps.atnl();
            },
            'x', 'y', 'g', 'v' => {
                // x without a pattern is every line (ecmd.c:568, linelooper).
                const n = ps.nextc();
                if (c != 'x' or (n != ' ' and n != '\t' and n != '\n')) {
                    _ = ps.skipbl();
                    const delim = ps.getch() orelse '\n';
                    if (delim == '\n') return fail(ps.why, "no address", .{});
                    try ps.okdelim(delim);
                    cmd.re = try ps.getregexp(delim);
                }
                if (ps.skipbl() == '\n') {
                    ps.i += 1;
                    const p = try ps.arena.create(Cmd);
                    p.* = .{ .c = 'p' };
                    cmd.sub = p;
                } else {
                    cmd.sub = (try ps.parse(nest)) orelse return fail(ps.why, "defcmd", .{});
                }
            },
            'm', 't' => {
                cmd.to = (try ps.address()) orelse return fail(ps.why, "bad address", .{});
                try ps.atnl();
            },
            'u' => {
                if (a != null) return fail(ps.why, "command takes no address", .{});
                cmd.num = ps.getnum(2);
                try ps.atnl();
            },
            '{' => {
                var last: ?*Cmd = null;
                while (true) {
                    if (ps.skipbl() == '\n') ps.i += 1;
                    const next = (try ps.parse(nest + 1)) orelse break;
                    if (last) |l| l.next = next else cmd.sub = next;
                    last = next;
                }
            },
            '}' => {
                try ps.atnl();
                if (nest == 0) return fail(ps.why, "right brace with no left brace", .{});
                return null;
            },
            'b', 'B', 'D', 'e', 'r', 'w', 'f', 'X', 'Y', '<', '|', '>' => return fail(ps.why, "{c} is not supported in pardes", .{c}),
            else => return fail(ps.why, "unknown command {c}", .{c}),
        }
        return cmd;
    }
};

// ---- running: ecmd.c, logging: elog.c ----

const Exec = struct {
    arena: std.mem.Allocator,
    text: []const u8,
    name: []const u8,
    why: *Why,
    ops: std.ArrayList(Op) = .empty,
    out: std.ArrayList(u8) = .empty,
    nest: u32 = 0,

    fn clip(n: usize) u32 {
        return std.math.cast(u32, n) orelse std.math.maxInt(u32);
    }

    /// An address from `dot`, or `dot` when there is none (aDot).
    fn address(ex: *Exec, expr: ?[]const u8, dot: Range) Failure!Range {
        const e = expr orelse return dot;
        var a: addr_lang.Addr = .{ .text = ex.text, .lim = null, .expr = e };
        const r = a.address(dot) orelse return fail(ex.why, "{s}", .{a.err});
        if (a.i < e.len) return fail(ex.why, "bad address syntax", .{});
        if (r.q0 > r.q1) return fail(ex.why, "addresses out of order", .{});
        return r;
    }

    /// elog.c's order: each change at or after the end of the one before,
    /// since they all address the text as it was.
    fn log(ex: *Exec, q0: u32, q1: u32, text: []const u8) Failure!void {
        if (q0 == q1 and text.len == 0) return;
        if (ex.ops.items.len > 0) {
            const prev = ex.ops.items[ex.ops.items.len - 1];
            if (q0 < prev.q1 or q0 < prev.q0) return fail(ex.why, "changes out of sequence", .{});
        }
        try ex.ops.append(ex.arena, .{ .q0 = q0, .q1 = q1, .text = text });
    }

    fn compile(ex: *Exec, pat: []const u8, c: u8) Failure!regexp.Regex {
        return regexp.Regex.compile(pat) catch fail(ex.why, "bad regexp in {c} command", .{c});
    }

    fn find(ex: *Exec, rx: *regexp.Regex, from: usize, hi: usize) Failure!?regexp.Regex.Match {
        if (from > hi) return null;
        return rx.find(ex.text, from, hi, hi) catch fail(ex.why, "{s}", .{addr_lang.e_slow});
    }

    /// ecmd.c:62, cmdexec: runs `c` from `dot`, answering the dot it leaves.
    fn exec(ex: *Exec, c: *Cmd, dot: Range) Failure!Range {
        const r = try ex.address(c.addr, dot);
        switch (c.c) {
            '\n' => return r,
            'a' => {
                try ex.log(r.q1, r.q1, c.text);
                return .{ .q0 = r.q1, .q1 = r.q1 };
            },
            'i' => {
                try ex.log(r.q0, r.q0, c.text);
                return .{ .q0 = r.q0, .q1 = r.q0 };
            },
            'c' => {
                try ex.log(r.q0, r.q1, c.text);
                return .{ .q0 = r.q0, .q1 = r.q0 };
            },
            'd' => {
                try ex.log(r.q0, r.q1, "");
                return .{ .q0 = r.q0, .q1 = r.q0 };
            },
            'p' => {
                try ex.out.appendSlice(ex.arena, ex.text[r.q0..r.q1]);
                return r;
            },
            '=' => {
                // ecmd.c:665, printposn
                if (c.text.len > 0) {
                    try ex.out.print(ex.arena, "{s}:#{d}", .{ ex.name, r.q0 });
                    if (r.q1 != r.q0) try ex.out.print(ex.arena, ",#{d}", .{r.q1});
                } else {
                    const l1 = 1 + std.mem.count(u8, ex.text[0..r.q0], "\n");
                    var l2 = l1 + std.mem.count(u8, ex.text[r.q0..r.q1], "\n");
                    if (r.q1 > r.q0 and ex.text[r.q1 - 1] == '\n') l2 -= 1;
                    try ex.out.print(ex.arena, "{s}:{d}", .{ ex.name, l1 });
                    if (l2 != l1) try ex.out.print(ex.arena, ",{d}", .{l2});
                }
                try ex.out.append(ex.arena, '\n');
                return r;
            },
            's' => return ex.substitute(c, r),
            'x', 'y' => {
                const ranges = if (c.re) |pat| try ex.looper(pat, r, c.c == 'x') else try ex.lines(r);
                ex.nest += 1;
                defer ex.nest -= 1;
                var d = r;
                for (ranges) |each| d = try ex.exec(c.sub.?, each);
                return d;
            },
            'g', 'v' => {
                var rx = try ex.compile(c.re.?, c.c);
                const hit = (try ex.find(&rx, r.q0, r.q1)) != null;
                if (hit != (c.c == 'v')) return ex.exec(c.sub.?, r);
                return r;
            },
            'm', 't' => {
                const to = try ex.address(c.to, dot);
                const bytes = ex.text[r.q0..r.q1];
                if (c.c == 't') {
                    try ex.log(to.q1, to.q1, bytes);
                } else if (r.q1 <= to.q0) {
                    // ecmd.c:408, move
                    try ex.log(r.q0, r.q1, "");
                    try ex.log(to.q1, to.q1, bytes);
                } else if (r.q0 >= to.q1) {
                    try ex.log(to.q1, to.q1, bytes);
                    try ex.log(r.q0, r.q1, "");
                } else if (!(r.q0 == to.q0 and r.q1 == to.q1)) {
                    return fail(ex.why, "move overlaps itself", .{});
                }
                return r;
            },
            'u' => return fail(ex.why, "u is only an Edit of its own here", .{}),
            '{' => {
                var it = c.sub;
                while (it) |each| : (it = each.next) _ = try ex.exec(each, r);
                return r;
            },
            else => return fail(ex.why, "unknown command {c}", .{c.c}),
        }
    }

    /// ecmd.c:443, s_cmd.
    fn substitute(ex: *Exec, c: *Cmd, r: Range) Failure!Range {
        var rx = try ex.compile(c.re.?, 's');
        var n = c.num;
        var op: ?usize = null;
        var p1: usize = r.q0;
        var did = false;
        while (p1 <= r.q1) {
            const m = (try ex.find(&rx, p1, r.q1)) orelse break;
            if (m.start == m.end) {
                if (op != null and m.start == op.?) {
                    p1 += 1;
                    continue;
                }
                p1 = m.end + 1;
            } else p1 = m.end;
            op = m.end;
            n -= 1;
            if (n > 0) continue;
            var buf: std.ArrayList(u8) = .empty;
            var i: usize = 0;
            while (i < c.text.len) : (i += 1) {
                const ch = c.text[i];
                if (ch == '\\' and i + 1 < c.text.len) {
                    i += 1;
                    const e = c.text[i];
                    if (e >= '1' and e <= '9') return fail(ex.why, "no \\{c}: mvzr keeps no submatches", .{e});
                    try buf.append(ex.arena, e);
                } else if (ch == '&') {
                    try buf.appendSlice(ex.arena, ex.text[m.start..m.end]);
                } else try buf.append(ex.arena, ch);
            }
            try ex.log(clip(m.start), clip(m.end), buf.items);
            did = true;
            if (!c.global) break;
        }
        if (!did and ex.nest == 0) return fail(ex.why, "no substitution", .{});
        return r;
    }

    /// ecmd.c:796, looper: the matches (x) or what lies between them (y).
    fn looper(ex: *Exec, pat: []const u8, r: Range, xy: bool) Failure![]Range {
        var rx = try ex.compile(pat, if (xy) 'x' else 'y');
        var out: std.ArrayList(Range) = .empty;
        var op: ?usize = if (xy) null else r.q0;
        var p: usize = r.q0;
        while (p <= r.q1) {
            var tr: Range = undefined;
            var sel_end: usize = undefined;
            if (try ex.find(&rx, p, r.q1)) |m| {
                if (m.start == m.end) {
                    if (op != null and m.start == op.?) {
                        p += 1;
                        continue;
                    }
                    p = m.end + 1;
                } else p = m.end;
                tr = if (xy) .{ .q0 = clip(m.start), .q1 = clip(m.end) } else .{ .q0 = clip(op.?), .q1 = clip(m.start) };
                sel_end = m.end;
            } else {
                // no match, but y still takes what is left
                if (xy or op.? > r.q1) break;
                tr = .{ .q0 = clip(op.?), .q1 = r.q1 };
                p = @as(usize, r.q1) + 1;
                sel_end = r.q1;
            }
            op = sel_end;
            try out.append(ex.arena, tr);
        }
        return out.items;
    }

    /// ecmd.c:840, linelooper: each line in the range, the first from where
    /// the range starts, the last cut where it ends.
    fn lines(ex: *Exec, r: Range) Failure![]Range {
        var out: std.ArrayList(Range) = .empty;
        var p: usize = r.q0;
        while (p < r.q1) {
            const nl = std.mem.indexOfScalarPos(u8, ex.text, p, '\n');
            const e = @min(if (nl) |i| i + 1 else ex.text.len, r.q1);
            if (e <= p) break;
            try out.append(ex.arena, .{ .q0 = clip(p), .q1 = clip(e) });
            p = e;
        }
        return out.items;
    }
};

/// The text after `ops`, which are in order and do not overlap.
pub fn apply(gpa: std.mem.Allocator, text: []const u8, ops: []const Op) ![]u8 {
    var n: usize = text.len;
    for (ops) |o| n = n - (o.q1 - o.q0) + o.text.len;
    const out = try gpa.alloc(u8, n);
    var at: usize = 0;
    var from: usize = 0;
    for (ops) |o| {
        const keep = text[from..o.q0];
        @memcpy(out[at..][0..keep.len], keep);
        at += keep.len;
        @memcpy(out[at..][0..o.text.len], o.text);
        at += o.text.len;
        from = o.q1;
    }
    @memcpy(out[at..], text[from..]);
    return out;
}

/// Where `dot` lands after `ops`: each end moved by the changes before it;
/// an empty dot where a change starts selects what it put there, as
/// elogapply does (elog.c:283).
pub fn moveDot(dot: Range, ops: []const Op) Range {
    var shift: i64 = 0;
    for (ops) |o| {
        if (dot.q0 == dot.q1 and o.q0 == dot.q0 and (o.text.len > 0)) {
            const q: u32 = @intCast(@as(i64, dot.q0) + shift);
            return .{ .q0 = q, .q1 = q + @as(u32, @intCast(o.text.len)) };
        }
        if (o.q1 > dot.q0) break;
        shift += @as(i64, @intCast(o.text.len)) - @as(i64, o.q1 - o.q0);
    }
    const q0: u32 = @intCast(@as(i64, dot.q0) + shift);
    var shift1: i64 = 0;
    for (ops) |o| {
        if (o.q1 > dot.q1 or (o.q0 == o.q1 and o.q0 == dot.q1)) break;
        shift1 += @as(i64, @intCast(o.text.len)) - @as(i64, o.q1 - o.q0);
    }
    const q1: u32 = @intCast(@max(@as(i64, q0), @as(i64, dot.q1) + shift1));
    return .{ .q0 = q0, .q1 = q1 };
}

fn edited(text: []const u8, command: []const u8) ![]u8 {
    var arena_state = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer arena_state.deinit();
    var why: Why = .{};
    const res = run(arena_state.allocator(), text, .{}, "t", command, &why) catch |err| {
        if (err == error.Edit) std.debug.print("Edit: {s}\n", .{why.text()});
        return err;
    };
    return apply(std.testing.allocator, text, res.ops);
}

fn expectEdit(text: []const u8, command: []const u8, want: []const u8) !void {
    const got = try edited(text, command);
    defer std.testing.allocator.free(got);
    try std.testing.expectEqualStrings(want, got);
}

test "sam's classic commands" {
    try expectEdit("foo x foo y foo\n", ",x/foo/c/foobar/", "foobar x foobar y foobar\n");
    try expectEdit("a b\nc d\n", ",x/ /c/_/", "a_b\nc_d\n");
    // As in acme (ecmd.c:810), y also takes the empty stretch after the last match.
    try expectEdit("one\ntwo\nthree\n", ",y/\\n/c/X/", "X\nX\nX\nX");
    try expectEdit("aaa\n", ",s/a/b/g", "bbb\n");
    try expectEdit("aaa\n", ",s/a/b/", "baa\n");
    try expectEdit("aaa\n", ",s2/a/b/", "aba\n");
    try expectEdit("cat dog\n", ",s/[a-z]+/<&>/g", "<cat> <dog>\n");
    // x within x, g within x.
    try expectEdit("int a;\nfloat b;\nint c;\n", ",x/.*\\n/g/^int/x/[a-z];/c/X;/", "int X;\nfloat b;\nint X;\n");
    try expectEdit("keep 1\ndrop 2\nkeep 3\n", ",x g/drop/d", "keep 1\nkeep 3\n");
    try expectEdit("keep 1\ndrop 2\nkeep 3\n", ",x v/keep/d", "keep 1\nkeep 3\n");
    // Lines, a/i, grouping, m and t.
    try expectEdit("a\nb\n", ",x a/;/", "a\n;b\n;");
    try expectEdit("a\nb\n", "1 i/>/", ">a\nb\n");
    try expectEdit("abc\n", "/b/{\ni/[/\na/]/\n}", "a[b]c\n");
    try expectEdit("one\ntwo\n", "1 m $", "two\none\n");
    try expectEdit("one\ntwo\n", "1 t $", "one\ntwo\none\n");
    try expectEdit("one\ntwo\nthree\n", "2 d", "one\nthree\n");
    // pardes's line:col.
    try expectEdit("one\ntwo\n", "2:2 i/-/", "one\nt-wo\n");
}

test "an Edit that fails halfway changes nothing, and says why in acme's words" {
    var arena_state = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer arena_state.deinit();
    for ([_][2][]const u8{
        .{ ",x/foo/c/bar/\n/nomatch/d", "no match for regexp" },
        .{ ",s/zzz/y/", "no substitution" },
        .{ "k", "unknown command k" },
        .{ ",x/a/{\na/1/\ni/2/\n}", "changes out of sequence" },
        .{ "}", "right brace with no left brace" },
        .{ "w /tmp/x", "w is not supported in pardes" },
        .{ ",s/(a)/\\1/", "no \\1: mvzr keeps no submatches" },
        .{ "1 m 1,2", "move overlaps itself" },
    }) |c| {
        var why: Why = .{};
        try std.testing.expectError(error.Edit, run(arena_state.allocator(), "foo a\nfoo\n", .{}, "t", c[0], &why));
        try std.testing.expectEqualStrings(c[1], why.text());
    }
}

test "the dot an Edit leaves selects what a change put where it stood" {
    const ops = [_]Op{.{ .q0 = 2, .q1 = 4, .text = "XYZ" }};
    try std.testing.expectEqual(Range{ .q0 = 2, .q1 = 5 }, moveDot(.{ .q0 = 2, .q1 = 2 }, &ops));
    try std.testing.expectEqual(Range{ .q0 = 6, .q1 = 7 }, moveDot(.{ .q0 = 5, .q1 = 6 }, &ops));
}