runner.go: Add unit tests for context shifting

This also makes it easier to truncate long inputs the same as
shifting but does not actually implement it. This type of
truncation has a trade off between quality and time to first
token.

llama/runner/cache_test.go

@@ -227,3 +227,66 @@ func TestFindCacheSlot(t *testing.T) {
 		})
 	}
 }
+
+func TestShiftDiscard(t *testing.T) {
+	tests := []struct {
+		name     string
+		numCtx   int
+		numKeep  int
+		inputLen int
+		expected int
+	}{
+		{
+			name:     "Shift",
+			numCtx:   2048,
+			numKeep:  5,
+			inputLen: 2048,
+			expected: 1021,
+		},
+		{
+			name:     "Max Keep",
+			numCtx:   2048,
+			numKeep:  2047,
+			inputLen: 2048,
+			expected: 1,
+		},
+		{
+			name:     "No Keep",
+			numCtx:   2048,
+			numKeep:  0,
+			inputLen: 2048,
+			expected: 1024,
+		},
+		{
+			name:     "Truncate",
+			numCtx:   2048,
+			numKeep:  5,
+			inputLen: 5000,
+			expected: 3973,
+		},
+		{
+			name:     "Truncate Keep",
+			numCtx:   2048,
+			numKeep:  2047,
+			inputLen: 5000,
+			expected: 2953,
+		},
+		{
+			name:     "No Op",
+			numCtx:   2048,
+			numKeep:  5,
+			inputLen: 512,
+			expected: 0,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			c := InputCache{numCtx: tt.numCtx}
+			result := c.ShiftDiscard(tt.inputLen, tt.numKeep)
+			if result != tt.expected {
+				t.Errorf("shiftDiscard(ctx: %v, keep: %v input: %v): have %v; want %v", tt.numCtx, tt.numKeep, tt.inputLen, result, tt.expected)
+			}
+		})
+	}
+}