feat(utils): introduce EventStateMachine

Ref: https://github.com/fcitx5-android/fcitx5-android/blob/b381b414312e4180416dcb1d35b7339d575e6c94/app/src/main/java/org/fcitx/fcitx5/android/utils/EventStateMachine.kt

app/src/main/java/com/osfans/trime/util/EventStateMachine.kt

@@ -0,0 +1,148 @@
+// SPDX-FileCopyrightText: 2015 - 2024 Rime community
+//
+// SPDX-License-Identifier: GPL-3.0-or-later
+package com.osfans.trime.util
+
+/**
+ * Adapted from fcitx5-android.
+ *
+ * @see <a href="https://github.com/fcitx5-android/fcitx5-android/blob/b381b414312e4180416dcb1d35b7339d575e6c94/app/src/main/java/org/fcitx/fcitx5/android/utils/EventStateMachine.kt">EventStateMachine</a>
+ */
+class EventStateMachine<State : Any, Event : EventStateMachine.TransitionEvent<State, B>, B : EventStateMachine.BooleanStateKey>(
+    private val initialState: State,
+    private val externalBooleanStates: MutableMap<B, Boolean>,
+) {
+    interface BooleanStateKey {
+        val name: String
+    }
+
+    interface TransitionEvent<State : Any, B : BooleanStateKey> {
+        /**
+         * INVARIANT: No side effects
+         * @return the next state
+         */
+        fun accept(
+            initialState: State,
+            currentState: State,
+            useBoolean: (B) -> Boolean?,
+        ): State
+    }
+
+    var onNewStateListener: ((State) -> Unit)? = null
+
+    var currentState = initialState
+        private set
+
+    /**
+     * Push an event that may trigger a transition of state
+     */
+    fun push(event: Event) {
+        val newState = event.accept(initialState, currentState) { externalBooleanStates[it] }
+        if (newState == currentState) {
+            return
+        }
+        currentState = newState
+        onNewStateListener?.invoke(newState)
+    }
+
+    /**
+     * Update boolean states and push an event
+     */
+    fun push(
+        event: Event,
+        vararg booleanStates: Pair<B, Boolean>,
+    ) {
+        booleanStates.forEach {
+            setBooleanState(it.first, it.second)
+        }
+        push(event)
+    }
+
+    fun setBooleanState(
+        key: B,
+        value: Boolean,
+    ) {
+        externalBooleanStates[key] = value
+    }
+
+    fun getBooleanState(key: B) = externalBooleanStates[key]
+
+    fun unsafeJump(state: State) {
+        currentState = state
+        onNewStateListener?.invoke(state)
+    }
+}
+
+// DSL
+class TransitionEventBuilder<State : Any, B : EventStateMachine.BooleanStateKey> {
+    private var raw: ((State, State, (B) -> Boolean?) -> State)? = null
+
+    inner class Builder(val source: State) {
+        lateinit var target: State
+        var pred: ((B) -> Boolean?) -> Boolean = { _ -> true }
+    }
+
+    infix fun Builder.transitTo(state: State) =
+        apply {
+            target = state
+        }
+
+    infix fun Builder.on(expected: Pair<B, Boolean>) =
+        apply {
+            this.pred = { it(expected.first) == expected.second }
+        }
+
+    infix fun Builder.onF(pred: ((B) -> Boolean?) -> Boolean) =
+        apply {
+            this.pred = pred
+        }
+
+    val builders = mutableListOf<Builder>()
+
+    fun from(state: State) = Builder(state).also { builders += it }
+
+    /**
+     * Use either [from] or [accept] to build the transition event
+     */
+    fun accept(block: ((State, State, (B) -> Boolean?) -> State)) =
+        apply {
+            raw = block
+        }
+
+    fun build() =
+        object : EventStateMachine.TransitionEvent<State, B> {
+            override fun accept(
+                initialState: State,
+                currentState: State,
+                useBoolean: (B) -> Boolean?,
+            ): State {
+                if (raw != null) {
+                    return raw!!(initialState, currentState, useBoolean)
+                }
+                val filtered = builders.filter { it.source == currentState && it.pred(useBoolean) }
+                return when (filtered.size) {
+                    0 -> currentState
+                    1 -> filtered[0].target
+                    else -> {
+                        val first = filtered[0].target
+                        first
+                    }
+                }
+            }
+        }
+}
+
+typealias TransitionBuildBlock<State, B> = TransitionEventBuilder<State, B>.() -> Unit
+
+class BuildTransitionEvent<State : Any, B : EventStateMachine.BooleanStateKey>(block: TransitionBuildBlock<State, B>) :
+    EventStateMachine.TransitionEvent<State, B> {
+    private val delegate: EventStateMachine.TransitionEvent<State, B> by lazy {
+        TransitionEventBuilder<State, B>().also(block).build()
+    }
+
+    override fun accept(
+        initialState: State,
+        currentState: State,
+        useBoolean: (B) -> Boolean?,
+    ): State = delegate.accept(initialState, currentState, useBoolean)
+}