Update the boundaries

__tests__/algorithm.test.ts

@@ -499,6 +499,15 @@ describe('next_state', () => {
       )
     }).toThrow('invalid memory state')
 
+    // t<0
+    expect(() => {
+      f.next_state(
+        { stability: 0, difficulty: 0 },
+        -1 /** invalid delta_t */,
+        1 /** Again */
+      )
+    }).toThrow('invalid memory state')
+
     // g<0
     expect(() => {
       f.next_state(init, 1, -1 /** invalid grade */)

src/fsrs/algorithm.ts

@@ -289,21 +289,24 @@ export class FSRSAlgorithm {
       difficulty: 0,
       stability: 0,
     }
+    if (g < 0 || g > 4 || t < 0) {
+      throw new Error('invalid memory state')
+    }
     if (d === 0 && s === 0) {
       return {
-        difficulty: this.init_difficulty(clamp(g, 1, 4)),
-        stability: this.init_stability(clamp(g, 1, 4)),
+        difficulty: this.init_difficulty(g),
+        stability: this.init_stability(g),
       }
     }
-    if (d < 1 || s < 0.01 || g < 0 || g > 4) {
-      throw new Error('invalid memory state')
-    }
     if (g === 0) {
       return {
         difficulty: d,
         stability: s,
       }
     }
+    if (d < 1 || s < 0.01) {
+      throw new Error('invalid memory state')
+    }
     const r = this.forgetting_curve(t, s)
     const s_after_success = this.next_recall_stability(d, s, r, g)
     const s_after_fail = this.next_forget_stability(d, s, r)