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import type {
KeyboardModifiers,
ResolvedKey,
VirtualKeyEvent,
} from "../types";
/** A caret/selection-aware editing snapshot. */
export interface EditState {
value: string;
selectionStart: number;
selectionEnd: number;
}
/** Behavior options that constrain an edit. */
export interface ApplyKeyOptions {
/** Maximum length of the value; inserts that would exceed it are ignored. */
maxLength?: number;
/** Return `false` to reject an insertion. Receives the inserted text and the candidate value. */
keyFilter?: (key: string, nextValue: string) => boolean;
/** When true, Enter fires a submit instead of inserting a newline. */
submitOnEnter?: boolean;
}
/** The result of applying a key to an {@link EditState}. */
export interface ApplyKeyResult {
/** The value after the edit (unchanged when `changed` is false). */
value: string;
/** Caret/selection start after the edit. */
selectionStart: number;
/** Caret/selection end after the edit. */
selectionEnd: number;
/** Whether the value actually changed. */
changed: boolean;
/** Set when Enter fired as a submit (`submitOnEnter`). */
submit: boolean;
/** The synthetic event describing this press. */
event: VirtualKeyEvent;
}
function clamp(n: number, min: number, max: number): number {
Iif (Number.isNaN(n)) return min;
return Math.max(min, Math.min(max, n));
}
/**
* The number of UTF-16 code units to delete for a single "backspace" ending at
* `index`. Removes a full surrogate pair (e.g. an emoji) as one unit.
*/
function backspaceUnits(value: string, index: number): number {
Iif (index <= 0) return 0;
const prev = value.charCodeAt(index - 1);
// Low surrogate preceded by a high surrogate → delete both.
if (prev >= 0xdc00 && prev <= 0xdfff && index >= 2) {
const prev2 = value.charCodeAt(index - 2);
Eif (prev2 >= 0xd800 && prev2 <= 0xdbff) return 2;
}
return 1;
}
/**
* Apply a resolved key press to an editing snapshot, returning the next value
* and caret position. Pure and DOM-free, so it is trivially unit-testable and
* SSR-safe.
*
* Handles character insertion (respecting `maxLength` / `keyFilter`), Space,
* Tab, Enter (newline or submit), Backspace (deletes the selection, else the
* char before the caret), and Clear. Modifier / layout-switch / hide actions
* are no-ops here (the hook handles those) and return `changed: false`.
*
* @example
* ```ts
* const key = resolveKey({ key: "a" }, mods);
* applyKey(key, { value: "bc", selectionStart: 1, selectionEnd: 1 }, mods);
* // → { value: "bac", selectionStart: 2, selectionEnd: 2, changed: true, ... }
* ```
*/
export function applyKey(
key: ResolvedKey,
state: EditState,
modifiers: KeyboardModifiers,
options: ApplyKeyOptions = {}
): ApplyKeyResult {
const event = buildKeyEvent(key, modifiers);
const { value } = state;
const len = value.length;
const start = clamp(state.selectionStart, 0, len);
const end = clamp(state.selectionEnd, start, len);
const before = value.slice(0, start);
const afterSelection = value.slice(end);
const noChange: ApplyKeyResult = {
value,
selectionStart: start,
selectionEnd: end,
changed: false,
submit: false,
event,
};
const action = key.action;
// Modifier / structural actions do not touch the value here.
if (
action === "shift" ||
action === "capslock" ||
action === "layer" ||
action === "layout-switch" ||
action === "hide"
) {
return noChange;
}
if (action === "clear") {
if (value === "") return noChange;
return {
value: "",
selectionStart: 0,
selectionEnd: 0,
changed: true,
submit: false,
event,
};
}
if (action === "enter" && options.submitOnEnter) {
return { ...noChange, submit: true };
}
if (action === "backspace") {
if (start !== end) {
// Delete the active selection.
const next = before + afterSelection;
return {
value: next,
selectionStart: start,
selectionEnd: start,
changed: true,
submit: false,
event,
};
}
if (start === 0) return noChange;
const units = backspaceUnits(value, start);
const caret = start - units;
return {
value: value.slice(0, caret) + afterSelection,
selectionStart: caret,
selectionEnd: caret,
changed: true,
submit: false,
event,
};
}
// Everything else inserts text: char keys, Space, Tab, Enter (newline).
let text: string;
if (action === "space") text = " ";
else if (action === "tab") text = "\t";
else if (action === "enter") text = "\n";
else text = key.effectiveValue;
if (text === "") return noChange;
const candidate = before + text + afterSelection;
if (options.maxLength !== undefined && candidate.length > options.maxLength) {
return noChange;
}
if (options.keyFilter && !options.keyFilter(text, candidate)) {
return noChange;
}
const caret = start + text.length;
return {
value: candidate,
selectionStart: caret,
selectionEnd: caret,
changed: true,
submit: false,
event,
};
}
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