//! The core component contract for the TUI.
//!
//! Dispatch is a vtable of function pointers over `*anyopaque` (decided in
//! plan §4.5; a tagged union was rejected so out-of-tree extensions can define
//! their own components later without editing a central enum).
//!
//! The render engine (later sub-phase) holds a list of `Component`s, asks each
//! to `render(width)` into lines, and uses `firstLineChanged` to do a
//! differential repaint. This file defines only the interface plus a small
//! reusable cache/dirty mixin; it implements no concrete components and no
//! engine.

const std = @import("std");

/// Invisible cursor marker.
///
/// A focused `Focusable` component embeds this marker in its rendered output
/// at the virtual cursor location. It is an APC string (`ESC _ ... ESC \`),
/// which terminals ignore (zero visible width), so it survives in the line
/// buffer until the engine scans for it. In P3 the engine will locate this
/// marker, strip it from the emitted bytes, and position the hardware cursor
/// there; for P1 the marker simply ships and is treated as zero-width.
///
/// The payload `panto-cursor` disambiguates it from any other APC a child
/// component might legitimately emit.
pub const CURSOR_MARKER = "\x1b_panto-cursor\x1b\\";

/// The component vtable. Concrete components store their own state behind
/// `ptr` and provide function pointers that recover the concrete type via
/// `@ptrCast`/`@alignCast`.
///
/// Line / width contract (plan §3.1): every line returned by `render` MUST
/// have visible width <= the `width` argument. The engine validates this and
/// treats overflow as a hard error, so components are responsible for
/// truncating. The returned `[]const []const u8` and its lines are owned by
/// the component (typically backed by its render cache); they must remain
/// valid until the next `render`/`invalidate` call on that component.
pub const Component = struct {
    ptr: *anyopaque,
    vtable: *const VTable,

    pub const VTable = struct {
        /// Render the component at `width` columns, returning one slice per
        /// visible line. Each line's visible width must be <= `width`.
        /// `alloc` is the engine's per-frame/render allocator; whether the
        /// component caches into its own storage or allocates fresh each call
        /// is up to it, but the returned slices must outlive the call until
        /// the next render/invalidate.
        render: *const fn (ptr: *anyopaque, width: usize, alloc: std.mem.Allocator) anyerror![]const []const u8,

        /// Lowest component-local line index whose rendered output differs
        /// since the last successful render, or null if nothing changed.
        ///
        /// This MUST be derived from the component's render cache (see
        /// `RenderCache`), not a separately hand-managed integer that can
        /// drift from reality.
        firstLineChanged: *const fn (ptr: *anyopaque) ?usize,

        /// Drop cached render state, forcing a full re-render and re-dirtying
        /// the component.
        invalidate: *const fn (ptr: *anyopaque) void,

        /// Optional: feed raw input bytes (already routed to this component by
        /// the engine) for the component to interpret. Null when the component
        /// does not accept input.
        handleInput: ?*const fn (ptr: *anyopaque, data: []const u8) void = null,

        /// Capability: when true the engine should deliver key *release*
        /// events to this component (most components only want press/repeat).
        wantsKeyRelease: bool = false,
    };

    pub fn render(self: Component, width: usize, alloc: std.mem.Allocator) anyerror![]const []const u8 {
        return self.vtable.render(self.ptr, width, alloc);
    }

    pub fn firstLineChanged(self: Component) ?usize {
        return self.vtable.firstLineChanged(self.ptr);
    }

    pub fn invalidate(self: Component) void {
        self.vtable.invalidate(self.ptr);
    }

    pub fn handleInput(self: Component, data: []const u8) void {
        if (self.vtable.handleInput) |f| f(self.ptr, data);
    }

    pub fn wantsKeyRelease(self: Component) bool {
        return self.vtable.wantsKeyRelease;
    }
};

/// The focus contract. A component that can hold focus exposes `focused`,
/// which the engine flips on focus changes. When focused, the component emits
/// `CURSOR_MARKER` at its virtual cursor position inside its `render` output.
///
/// This is a thin embeddable struct: a focusable component contains one and
/// the engine sets `.focused` via the component's own setter (the vtable does
/// not yet carry focus methods — focus routing wiring is part of the engine
/// sub-phase; what ships in P1 is the flag + the marker constant + the
/// documented contract).
pub const Focusable = struct {
    focused: bool = false,

    pub fn setFocused(self: *Focusable, value: bool) void {
        self.focused = value;
    }
};

/// Reusable cache + dirty bookkeeping for components.
///
/// firstLineChanged lifecycle (mirrors pi's `invalidate()` model):
///   - State mutation calls `markDirty()` (or `invalidate()`), which clears
///     the cache. While dirty, `firstLineChanged()` reports the recorded
///     dirty line (0 by default — "everything from the top changed").
///   - A *successful* render calls `store(lines)`, which:
///       1. diffs the new lines against the previously cached lines,
///       2. records the lowest differing index as the "changed-from" value,
///       3. replaces the cache with a copy of the new lines,
///       4. marks the cache clean.
///   - Once the cache is CLEAN, `firstLineChanged()` returns null regardless
///     of the diff `store` recorded. `firstLineChanged()` is the LIVE
///     pre-render dirty signal (the engine reads it once per slot BEFORE
///     `render` to decide whether to re-render and where to cut); a clean
///     component is already up to date on screen and has nothing pending, so
///     it must report null. The recorded diff index is retained separately in
///     `changed_from` as render bookkeeping and is NOT a live signal — exposing
///     it while clean would peg the differential cut to a stale line on every
///     later frame (a first-paint store records `changed_from == 0`, which
///     would otherwise force the cut to line 0 forever).
///
/// Because `firstLineChanged` is computed purely from cache state, it cannot
/// drift from a hand-managed field. Components embed this struct and route
/// their vtable through `cacheRender`/`markDirty`.
///
/// The cache owns its copies of the line bytes (allocated with the allocator
/// passed to `init`); call `deinit` to free them.
pub const RenderCache = struct {
    alloc: std.mem.Allocator,
    /// Owned copies of the last rendered lines, or null when never rendered or
    /// invalidated.
    lines: ?[][]u8 = null,
    /// Lowest line index that changed at the last `store` (or the pending
    /// dirty line while dirty; 0 when dirty with no prior cache). This is
    /// render bookkeeping, NOT the live signal: when the cache is clean,
    /// `firstLineChanged()` returns null even though this still holds the last
    /// diff index. Null only when a clean store found no change.
    changed_from: ?usize = null,
    dirty: bool = true,

    pub fn init(alloc: std.mem.Allocator) RenderCache {
        return .{ .alloc = alloc };
    }

    pub fn deinit(self: *RenderCache) void {
        self.freeLines();
    }

    fn freeLines(self: *RenderCache) void {
        if (self.lines) |lines| {
            for (lines) |line| self.alloc.free(line);
            self.alloc.free(lines);
            self.lines = null;
        }
    }

    /// Mark the component dirty (e.g. on any state mutation). Drops the cache
    /// so the next render is a full render starting at `from` (default 0).
    pub fn markDirty(self: *RenderCache) void {
        self.markDirtyFrom(0);
    }

    /// Mark dirty starting at a specific line. The cache is dropped (we no
    /// longer trust any cached line), and `firstLineChanged` will report
    /// `from` until the next successful render.
    pub fn markDirtyFrom(self: *RenderCache, from: usize) void {
        self.dirty = true;
        self.changed_from = from;
        self.freeLines();
    }

    /// Mark dirty for an APPEND-style mutation that only affects the tail of
    /// the rendered output (e.g. a streaming content delta appended to the
    /// end of a buffer).
    ///
    /// Unlike `markDirty`/`markDirtyFrom`, this RETAINS the cached baseline
    /// lines so the next `store` can diff against them and recover the TRUE
    /// lowest-changed index (which, for an append, is near the tail). While
    /// dirty, `firstLineChanged` reports a conservative TAIL HINT — the index
    /// of the last cached line (or 0 when there is no baseline yet) — so the
    /// engine re-renders and the cut stays near the end rather than rolling to
    /// line 0. The post-render diff in `store` then replaces the hint with the
    /// exact change point. `firstLineChanged` therefore remains cache-derived:
    /// the dirty hint comes from the retained cache's length, and the precise
    /// value comes from the diff.
    pub fn markDirtyAppend(self: *RenderCache) void {
        self.dirty = true;
        if (self.lines) |lines| {
            // Tail hint: the last existing line is the lowest line an append
            // can change. Keep the baseline for the diff in `store`.
            self.changed_from = if (lines.len == 0) 0 else lines.len - 1;
        } else {
            self.changed_from = 0;
        }
    }

    /// Equivalent to `markDirty` — full drop + re-dirty. Provided so a
    /// component's `invalidate` vtable entry can forward here verbatim.
    pub fn invalidate(self: *RenderCache) void {
        self.markDirty();
    }

    /// Derived dirty signal. Returns the lowest changed line index, or null
    /// when the cache is clean and nothing changed at the last store.
    pub fn firstLineChanged(self: *const RenderCache) ?usize {
        // The engine reads this as the PRE-render dirty signal (once per slot,
        // before `render`). A clean cache has no pending change to repaint, so
        // it must report null — even though the last `store` recorded a
        // `changed_from` diff index. Returning that retained index while clean
        // makes an unchanged component spuriously cut the differential repaint
        // at its stale diff line on EVERY later frame (notably a first-paint
        // store leaves `changed_from == 0`, which would peg the cut to line 0
        // forever). The retained `changed_from` is internal render bookkeeping,
        // not a live signal; only the dirty state drives a repaint.
        if (self.dirty) return self.changed_from orelse 0;
        return null;
    }

    /// Record a successful render. Diffs `new_lines` against the cache,
    /// updates `changed_from` to the lowest differing index, copies the new
    /// lines into the cache, and marks it clean.
    pub fn store(self: *RenderCache, new_lines: []const []const u8) !void {
        const diff = self.computeFirstDiff(new_lines);

        // Build the new owned copy first; only swap in on success.
        var copies = try self.alloc.alloc([]u8, new_lines.len);
        var made: usize = 0;
        errdefer {
            for (copies[0..made]) |c| self.alloc.free(c);
            self.alloc.free(copies);
        }
        for (new_lines, 0..) |line, i| {
            copies[i] = try self.alloc.dupe(u8, line);
            made = i + 1;
        }

        self.freeLines();
        self.lines = copies;
        self.changed_from = diff;
        self.dirty = false;
    }

    /// Lowest index at which `new_lines` differs from the cached lines.
    /// Returns null when they are identical. When there is no prior cache (or
    /// the line counts differ at index 0), returns 0.
    fn computeFirstDiff(self: *const RenderCache, new_lines: []const []const u8) ?usize {
        const old = self.lines orelse return 0;
        const n = @min(old.len, new_lines.len);
        var i: usize = 0;
        while (i < n) : (i += 1) {
            if (!std.mem.eql(u8, old[i], new_lines[i])) return i;
        }
        // Common prefix matched; if lengths differ, the first extra/missing
        // line index is the change point.
        if (old.len != new_lines.len) return n;
        return null;
    }
};

test "CURSOR_MARKER is an APC string" {
    try std.testing.expect(std.mem.startsWith(u8, CURSOR_MARKER, "\x1b_"));
    try std.testing.expect(std.mem.endsWith(u8, CURSOR_MARKER, "\x1b\\"));
}

test "RenderCache: starts dirty from 0" {
    var c = RenderCache.init(std.testing.allocator);
    defer c.deinit();
    try std.testing.expectEqual(@as(?usize, 0), c.firstLineChanged());
}

test "RenderCache: store cleans, no-change render returns null" {
    var c = RenderCache.init(std.testing.allocator);
    defer c.deinit();

    const a = [_][]const u8{ "one", "two" };
    try c.store(&a);
    // store cleans the cache, so the live signal is null (clean => null).
    // The recorded diff (first store after empty => 0) lives in changed_from.
    try std.testing.expectEqual(@as(?usize, null), c.firstLineChanged());
    try std.testing.expectEqual(@as(?usize, 0), c.changed_from);

    // Re-store identical => no change, still clean.
    try c.store(&a);
    try std.testing.expectEqual(@as(?usize, null), c.firstLineChanged());
    try std.testing.expectEqual(@as(?usize, null), c.changed_from);
}

test "RenderCache: store reports lowest differing line" {
    var c = RenderCache.init(std.testing.allocator);
    defer c.deinit();

    const a = [_][]const u8{ "one", "two", "three" };
    try c.store(&a);
    const b = [_][]const u8{ "one", "TWO", "three" };
    try c.store(&b);
    // The diff store computed is recorded in changed_from; the live signal is
    // null because the cache is now clean (already rendered on screen).
    try std.testing.expectEqual(@as(?usize, 1), c.changed_from);
    try std.testing.expectEqual(@as(?usize, null), c.firstLineChanged());
}

test "RenderCache: line-count change reports the boundary" {
    var c = RenderCache.init(std.testing.allocator);
    defer c.deinit();

    const a = [_][]const u8{ "one", "two" };
    try c.store(&a);
    const b = [_][]const u8{ "one", "two", "three" };
    try c.store(&b);
    try std.testing.expectEqual(@as(?usize, 2), c.changed_from);
    try std.testing.expectEqual(@as(?usize, null), c.firstLineChanged());
}

test "RenderCache: markDirtyAppend keeps baseline and reports a tail hint, diff recovers exact change" {
    var c = RenderCache.init(std.testing.allocator);
    defer c.deinit();

    const a = [_][]const u8{ "l0", "l1", "l2", "l3" };
    try c.store(&a);
    // No baseline change yet; append-dirty reports the tail hint (last line).
    c.markDirtyAppend();
    try std.testing.expectEqual(@as(?usize, 3), c.firstLineChanged());
    // The baseline is retained for the diff.
    try std.testing.expect(c.lines != null);

    // A render that only adds a tail line: diff recovers the exact boundary (4),
    // NOT 0 — the streaming-tail property. The cache is clean after store, so
    // the recorded diff lives in changed_from and the live signal is null.
    const b = [_][]const u8{ "l0", "l1", "l2", "l3", "l4" };
    try c.store(&b);
    try std.testing.expectEqual(@as(?usize, 4), c.changed_from);
    try std.testing.expectEqual(@as(?usize, null), c.firstLineChanged());
}

test "RenderCache: markDirtyAppend with no baseline reports 0" {
    var c = RenderCache.init(std.testing.allocator);
    defer c.deinit();
    c.markDirtyAppend();
    try std.testing.expectEqual(@as(?usize, 0), c.firstLineChanged());
}

test "RenderCache: markDirty re-dirties from 0" {
    var c = RenderCache.init(std.testing.allocator);
    defer c.deinit();
    const a = [_][]const u8{"x"};
    try c.store(&a);
    c.markDirty();
    try std.testing.expectEqual(@as(?usize, 0), c.firstLineChanged());
}

test "Focusable flips" {
    var f: Focusable = .{};
    try std.testing.expect(!f.focused);
    f.setFocused(true);
    try std.testing.expect(f.focused);
}