neovm-core 0.0.2

//! Undo system for buffers — GNU Emacs–compatible Lisp list approach.
//!
//! The undo list is stored as a direct Lisp `Value` in the buffer-local
//! property `buffer-undo-list`.  This module provides helper functions
//! that manipulate that `Value` list, matching GNU Emacs's undo.c:
//!
//! - `t` means undo is disabled
//! - `nil` means undo is enabled with an empty list
//! - Records are cons-ed onto the FRONT (most recent first)
//!
//! Entry types:
//! - `(BEG . END)` — insertion (1-indexed positions)
//! - `(TEXT . POS)` — deletion (TEXT is string, POS is 1-indexed,
//!    negative if point was at end of deleted region)
//! - `POS` (integer) — cursor position (1-indexed)
//! - `(t . MODTIME)` — first-change marker
//! - `nil` — undo boundary

use crate::emacs_core::value::{Value, ValueKind};
use crate::heap_types::LispString;

/// Returns `true` when `buffer-undo-list` is `t` (undo disabled).
pub fn undo_list_is_disabled(undo_list: &Value) -> bool {
    undo_list.is_t()
}

/// Record that text was inserted at byte position `beg` with byte length
/// `len`.  Positions stored in the list are 1-indexed.
///
/// If we are right at an undo boundary (head is nil or list is empty)
/// and `pt` != `beg`, a cursor-position entry is recorded first.
///
/// Consecutive adjacent inserts are merged when the head entry is an
/// insert whose END equals `beg+1` (the 1-indexed start of the new
/// insert).
pub fn undo_list_record_insert(undo_list: &mut Value, beg: usize, len: usize, pt: usize) {
    if undo_list_is_disabled(undo_list) || len == 0 {
        return;
    }

    let at_boundary = undo_list.is_nil() || (undo_list.is_cons() && undo_list.cons_car().is_nil());
    if at_boundary && pt != beg {
        undo_list_record_point(undo_list, pt);
    }

    let beg1 = (beg + 1) as i64;
    let end1 = (beg + len + 1) as i64;

    // Try to merge with the head entry if it's an adjacent insert.
    if undo_list.is_cons() {
        let head = undo_list.cons_car();
        if head.is_cons() {
            let car = head.cons_car();
            let cdr = head.cons_cdr();
            if let (Some(prev_beg), Some(prev_end)) = (car.as_fixnum(), cdr.as_fixnum()) {
                if prev_end == beg1 {
                    // Merge: extend the existing insert entry.
                    head.set_cdr(Value::fixnum(prev_end + len as i64));
                    return;
                }
                // Check if insert is at the beginning of the previous range
                if prev_beg == end1 {
                    head.set_car(Value::fixnum(beg1));
                    return;
                }
                let _ = (prev_beg, prev_end); // suppress unused warnings
            }
        }
    }

    let entry = Value::cons(Value::fixnum(beg1), Value::fixnum(end1));
    *undo_list = Value::cons(entry, *undo_list);
}

/// Record a deletion.  `beg` is the 0-indexed byte position, `text` is
/// the deleted string, `pt` is the 0-indexed cursor byte position at
/// the time of deletion.
///
/// The stored position is 1-indexed and negative when `pt` was at the
/// END of the deleted region (i.e. `pt == beg + text.len()`).
pub fn undo_list_record_delete(undo_list: &mut Value, beg: usize, text: LispString, pt: usize) {
    if undo_list_is_disabled(undo_list) || text.is_empty() {
        return;
    }

    let at_boundary = undo_list.is_nil() || (undo_list.is_cons() && undo_list.cons_car().is_nil());
    if at_boundary && pt != beg {
        undo_list_record_point(undo_list, pt);
    }

    let pos1 = (beg + 1) as i64;
    let stored_pos = if pt == beg + text.sbytes() {
        -pos1
    } else {
        pos1
    };

    let entry = Value::cons(Value::heap_string(text), Value::fixnum(stored_pos));
    *undo_list = Value::cons(entry, *undo_list);
}

/// Record the cursor position (0-indexed `pt`) as a 1-indexed integer.
/// Skips if the most recent entry is the same position.
pub fn undo_list_record_point(undo_list: &mut Value, pt: usize) {
    if undo_list_is_disabled(undo_list) {
        return;
    }
    let pt1 = Value::fixnum((pt + 1) as i64);

    // Don't record consecutive identical positions.
    if undo_list.is_cons() {
        let head = undo_list.cons_car();
        if head == pt1 {
            return;
        }
    }

    *undo_list = Value::cons(pt1, *undo_list);
}

/// Record a text-property change: `(nil PROP VAL BEG . END)`.
///
/// `prop` is the property name (symbol), `val` is the OLD value before
/// the change (so that undoing restores it), `beg` and `end` are
/// 0-indexed byte positions; they are stored as 1-indexed integers.
pub fn undo_list_record_property_change(
    undo_list: &mut Value,
    prop: Value,
    val: Value,
    beg: usize,
    end: usize,
) {
    if undo_list_is_disabled(undo_list) || beg >= end {
        return;
    }
    let beg1 = Value::fixnum((beg + 1) as i64);
    let end1 = Value::fixnum((end + 1) as i64);
    // Build (nil PROP VAL BEG . END)
    let inner = Value::cons(beg1, end1);
    let inner = Value::cons(val, inner);
    let inner = Value::cons(prop, inner);
    let entry = Value::cons(Value::NIL, inner);
    *undo_list = Value::cons(entry, *undo_list);
}

/// Record the first-change sentinel `(t . 0)`.
pub fn undo_list_record_first_change(undo_list: &mut Value) {
    if undo_list_is_disabled(undo_list) {
        return;
    }
    let entry = Value::cons(Value::T, Value::fixnum(0));
    *undo_list = Value::cons(entry, *undo_list);
}

/// Insert an undo boundary (`nil`).  Skips if the list is empty/nil or
/// already starts with a nil boundary.
pub fn undo_list_boundary(undo_list: &mut Value) {
    if undo_list_is_disabled(undo_list) {
        return;
    }
    // Don't add boundary to empty list or if head is already nil.
    if undo_list.is_nil() {
        return;
    }
    if undo_list.is_cons() && undo_list.cons_car().is_nil() {
        return;
    }
    *undo_list = Value::cons(Value::NIL, *undo_list);
}

/// Pop one undo group from the front of the list.
///
/// Skips leading nil boundaries, then collects entries until the next
/// nil boundary (or end of list).  Returns the collected entries in
/// the order they were popped (most recent first).
///
/// Mutates `undo_list` in place to remove the consumed entries.
pub fn undo_list_pop_group(undo_list: &mut Value) -> Vec<Value> {
    // Skip leading boundaries.
    while undo_list.is_cons() && undo_list.cons_car().is_nil() {
        *undo_list = undo_list.cons_cdr();
    }

    let mut group = Vec::new();
    while undo_list.is_cons() {
        let head = undo_list.cons_car();
        if head.is_nil() {
            // Hit the next boundary — stop.
            break;
        }
        group.push(head);
        *undo_list = undo_list.cons_cdr();
    }
    group
}

/// Check whether the undo list is non-empty (has actual records, not
/// just nil).
pub fn undo_list_is_empty(undo_list: &Value) -> bool {
    undo_list.is_nil()
}

/// Check whether the undo list contains at least one nil boundary.
pub fn undo_list_contains_boundary(undo_list: &Value) -> bool {
    let mut cursor = *undo_list;
    while cursor.is_cons() {
        if cursor.cons_car().is_nil() {
            return true;
        }
        cursor = cursor.cons_cdr();
    }
    false
}

/// Check whether the most recent entry is a nil boundary.
pub fn undo_list_has_trailing_boundary(undo_list: &Value) -> bool {
    undo_list.is_cons() && undo_list.cons_car().is_nil()
}

/// Estimate the byte size of one undo entry for truncation purposes.
/// Each cons cell counts as 16 bytes; strings count their byte length.
fn undo_entry_size(entry: &Value) -> usize {
    match entry.kind() {
        ValueKind::Nil => 0,
        ValueKind::Fixnum(_) => 8,
        ValueKind::String => entry.as_lisp_string().map(|s| s.sbytes()).unwrap_or(8),
        _ if entry.is_cons() => {
            let car = entry.cons_car();
            let cdr = entry.cons_cdr();
            let car_size = match car.kind() {
                ValueKind::String => car.as_lisp_string().map(|s| s.sbytes()).unwrap_or(8),
                _ => 8,
            };
            let cdr_size = match cdr.kind() {
                ValueKind::String => cdr.as_lisp_string().map(|s| s.sbytes()).unwrap_or(8),
                _ => 8,
            };
            16 + car_size + cdr_size
        }
        _ => 8,
    }
}

/// Truncate an undo list to stay within size limits.
///
/// Walks the list counting approximate byte size.  After exceeding
/// `undo_limit`, looks for the next nil boundary to truncate at.
/// After exceeding `undo_strong_limit`, truncates immediately.
///
/// Returns the truncated list.
pub fn truncate_undo_list(undo_list: Value, undo_limit: usize, undo_strong_limit: usize) -> Value {
    if undo_list_is_disabled(&undo_list) || undo_list.is_nil() {
        return undo_list;
    }

    let mut total_size: usize = 0;
    let mut past_limit = false;
    let mut scan = undo_list;

    while scan.is_cons() {
        let entry = scan.cons_car();
        total_size += undo_entry_size(&entry) + 16; // 16 for the cons cell itself

        if total_size > undo_strong_limit {
            // Immediate truncation: cut here.
            scan.set_cdr(Value::NIL);
            return undo_list;
        }

        if total_size > undo_limit {
            past_limit = true;
        }

        if past_limit && entry.is_nil() {
            // Found a boundary past the limit — truncate after this boundary.
            scan.set_cdr(Value::NIL);
            return undo_list;
        }

        scan = scan.cons_cdr();
    }

    // Never exceeded any limit — return as-is.
    undo_list
}

// ===========================================================================
// Tests
// ===========================================================================

#[cfg(test)]
#[path = "undo_test.rs"]
mod tests;