termi 0.1.2

A modal terminal code editor written in Rust
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//! # Word motions
//!
//! **Purpose:** decide where "the next word" is.
//!
//! **Responsibility:** classify characters into word / punctuation / whitespace
//! runs and walk over them. Split out of `cursor` because word boundaries are a
//! self-contained text question with their own rules, and keeping them here
//! leaves `cursor` about *state* rather than *scanning*.
//!
//! **Public API:** [`next_word_start`], [`prev_word_start`], [`word_end`].

use crate::editor::cursor::Position;
use crate::editor::document::Document;

/// The three kinds of character run a word motion can be inside.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum Class {
    Whitespace,
    /// Identifier material: letters, digits and `_`.
    Word,
    /// Everything else — operators, brackets, quotes.
    Punctuation,
}

impl Class {
    fn of(ch: char) -> Self {
        if ch.is_whitespace() {
            Self::Whitespace
        } else if ch.is_alphanumeric() || ch == '_' {
            Self::Word
        } else {
            Self::Punctuation
        }
    }
}

/// Class of the character at a rope index, or `None` past the end.
fn class_at(doc: &Document, index: usize) -> Option<Class> {
    doc.text().get_char(index).map(Class::of)
}

/// Start of the next word, like vim's `w`.
///
/// Skips the run the cursor is currently in, then any whitespace.
#[must_use]
pub fn next_word_start(doc: &Document, from: Position) -> Position {
    let len = doc.len_chars();
    let mut index = doc.pos_to_char(from);

    if let Some(start_class) = class_at(doc, index)
        && start_class != Class::Whitespace
    {
        while class_at(doc, index) == Some(start_class) {
            index += 1;
        }
    }
    while index < len && class_at(doc, index) == Some(Class::Whitespace) {
        index += 1;
    }
    doc.char_to_pos(index.min(len))
}

/// Start of the previous word, like vim's `b`.
#[must_use]
pub fn prev_word_start(doc: &Document, from: Position) -> Position {
    let mut index = doc.pos_to_char(from);
    if index == 0 {
        return Position::ZERO;
    }
    index -= 1;

    while index > 0 && class_at(doc, index) == Some(Class::Whitespace) {
        index -= 1;
    }
    if let Some(class) = class_at(doc, index)
        && class != Class::Whitespace
    {
        while index > 0 && class_at(doc, index - 1) == Some(class) {
            index -= 1;
        }
    }
    doc.char_to_pos(index)
}

/// Last character of the current or next word, like vim's `e`.
#[must_use]
pub fn word_end(doc: &Document, from: Position) -> Position {
    let len = doc.len_chars();
    let mut index = doc.pos_to_char(from);
    if index + 1 >= len {
        return doc.char_to_pos(len.saturating_sub(1));
    }
    index += 1;

    while index < len && class_at(doc, index) == Some(Class::Whitespace) {
        index += 1;
    }
    if let Some(class) = class_at(doc, index) {
        while class_at(doc, index + 1) == Some(class) {
            index += 1;
        }
    }
    doc.char_to_pos(index.min(len.saturating_sub(1)))
}