From 4504469fb9114be1912e089e9c6770ed69c015a9 Mon Sep 17 00:00:00 2001
From: Peter Marshall <peterm@canva.com>
Date: Tue, 21 May 2024 16:45:43 +1000
Subject: [PATCH] Shrink ComputedValue using a bitfield

---
 .changeset/dirty-turtles-marry.md       |  5 +++
 packages/mobx/src/core/computedvalue.ts | 52 +++++++++++++++++++++++--
 2 files changed, 53 insertions(+), 4 deletions(-)
 create mode 100644 .changeset/dirty-turtles-marry.md

diff --git a/.changeset/dirty-turtles-marry.md b/.changeset/dirty-turtles-marry.md
new file mode 100644
index 000000000..696b077f1
--- /dev/null
+++ b/.changeset/dirty-turtles-marry.md
@@ -0,0 +1,5 @@
+---
+"mobx": patch
+---
+
+Shrink ComputedValue using a bitfield
diff --git a/packages/mobx/src/core/computedvalue.ts b/packages/mobx/src/core/computedvalue.ts
index a56d59525..dc8bed1eb 100644
--- a/packages/mobx/src/core/computedvalue.ts
+++ b/packages/mobx/src/core/computedvalue.ts
@@ -56,6 +56,18 @@ export type IComputedDidChange<T = any> = {
     oldValue: T | undefined
 }
 
+function getFlag(flags: number, mask: number) {
+    return !!(flags & mask)
+}
+function setFlag(flags: number, mask: number, newValue: boolean): number {
+    if (newValue) {
+        flags |= mask
+    } else {
+        flags &= ~mask
+    }
+    return flags
+}
+
 /**
  * A node in the state dependency root that observes other nodes, and can be observed itself.
  *
@@ -79,8 +91,6 @@ export class ComputedValue<T> implements IObservable, IComputedValue<T>, IDeriva
     dependenciesState_ = IDerivationState_.NOT_TRACKING_
     observing_: IObservable[] = [] // nodes we are looking at. Our value depends on these nodes
     newObserving_ = null // during tracking it's an array with new observed observers
-    isBeingObserved_ = false
-    isPendingUnobservation_: boolean = false
     observers_ = new Set<IDerivation>()
     diffValue_ = 0
     runId_ = 0
@@ -90,8 +100,13 @@ export class ComputedValue<T> implements IObservable, IComputedValue<T>, IDeriva
     protected value_: T | undefined | CaughtException = new CaughtException(null)
     name_: string
     triggeredBy_?: string
-    isComputing_: boolean = false // to check for cycles
-    isRunningSetter_: boolean = false
+
+    private static readonly isComputingMask = 0b0001
+    private static readonly isRunningSetterMask = 0b0010
+    private static readonly isBeingObservedMask = 0b0100
+    private static readonly isPendingUnobservationMask = 0b1000
+    private flags = 0b0000
+
     derivation: () => T // N.B: unminified as it is used by MST
     setter_?: (value: T) => void
     isTracing_: TraceMode = TraceMode.NONE
@@ -153,6 +168,35 @@ export class ComputedValue<T> implements IObservable, IComputedValue<T>, IDeriva
         }
     }
 
+    // to check for cycles
+    private get isComputing_(): boolean {
+        return getFlag(this.flags, ComputedValue.isComputingMask)
+    }
+    private set isComputing_(newValue: boolean) {
+        this.flags = setFlag(this.flags, ComputedValue.isComputingMask, newValue)
+    }
+
+    private get isRunningSetter_(): boolean {
+        return getFlag(this.flags, ComputedValue.isRunningSetterMask)
+    }
+    private set isRunningSetter_(newValue: boolean) {
+        this.flags = setFlag(this.flags, ComputedValue.isRunningSetterMask, newValue)
+    }
+
+    get isBeingObserved_(): boolean {
+        return getFlag(this.flags, ComputedValue.isBeingObservedMask)
+    }
+    set isBeingObserved_(newValue: boolean) {
+        this.flags = setFlag(this.flags, ComputedValue.isBeingObservedMask, newValue)
+    }
+
+    get isPendingUnobservation_(): boolean {
+        return getFlag(this.flags, ComputedValue.isPendingUnobservationMask)
+    }
+    set isPendingUnobservation_(newValue: boolean) {
+        this.flags = setFlag(this.flags, ComputedValue.isPendingUnobservationMask, newValue)
+    }
+
     /**
      * Returns the current value of this computed value.
      * Will evaluate its computation first if needed.