add a test case with large decimal amount

handlers/application/menuhandler.go

@@ -1830,12 +1830,12 @@ func (h *MenuHandlers) ValidateAmount(ctx context.Context, sym string, input []b
 		return res, fmt.Errorf("missing session")
 	}
 	flag_invalid_amount, _ := h.flagManager.GetFlag("flag_invalid_amount")
-	store := h.userdataStore
+	userStore := h.userdataStore
 
 	var balanceValue float64
 
 	// retrieve the active balance
-	activeBal, err := store.ReadEntry(ctx, sessionId, storedb.DATA_ACTIVE_BAL)
+	activeBal, err := userStore.ReadEntry(ctx, sessionId, storedb.DATA_ACTIVE_BAL)
 	if err != nil {
 		logg.ErrorCtxf(ctx, "failed to read activeBal entry with", "key", storedb.DATA_ACTIVE_BAL, "error", err)
 		return res, err
@@ -1862,11 +1862,14 @@ func (h *MenuHandlers) ValidateAmount(ctx context.Context, sym string, input []b
 	}
 
 	// Format the amount to 2 decimal places before saving (truncated)
-	cents := int((inputAmount + 1e-9) * 100)
-	truncated := float64(cents) / 100
-	formattedAmount := fmt.Sprintf("%.2f", truncated)
+	formattedAmount, err := store.TruncateDecimalString(amountStr, 2)
+	if err != nil {
+		res.FlagSet = append(res.FlagSet, flag_invalid_amount)
+		res.Content = amountStr
+		return res, nil
+	}
 
-	err = store.WriteEntry(ctx, sessionId, storedb.DATA_AMOUNT, []byte(formattedAmount))
+	err = userStore.WriteEntry(ctx, sessionId, storedb.DATA_AMOUNT, []byte(formattedAmount))
 	if err != nil {
 		logg.ErrorCtxf(ctx, "failed to write amount entry with", "key", storedb.DATA_AMOUNT, "value", formattedAmount, "error", err)
 		return res, err
@@ -3049,7 +3052,15 @@ func (h *MenuHandlers) SwapPreview(ctx context.Context, sym string, input []byte
 		return res, nil
 	}
 
-	finalAmountStr, err := store.ParseAndScaleAmount(inputStr, swapData.ActiveSwapFromDecimal)
+	// Format the amount to 2 decimal places
+	formattedAmount, err := store.TruncateDecimalString(inputStr, 2)
+	if err != nil {
+		res.FlagSet = append(res.FlagSet, flag_invalid_amount)
+		res.Content = inputStr
+		return res, nil
+	}
+
+	finalAmountStr, err := store.ParseAndScaleAmount(formattedAmount, swapData.ActiveSwapFromDecimal)
 	if err != nil {
 		return res, err
 	}

handlers/application/menuhandler_test.go

@@ -1686,6 +1686,14 @@ func TestValidateAmount(t *testing.T) {
 				Content: "0.14",
 			},
 		},
+		{
+			name:      "Test with valid large decimal amount",
+			input:     []byte("1.8599999999"),
+			activeBal: []byte("5"),
+			expectedResult: resource.Result{
+				Content: "1.85",
+			},
+		},
 	}
 
 	for _, tt := range tests {

store/tokens_test.go

@@ -7,6 +7,109 @@ import (
 	"github.com/alecthomas/assert/v2"
 )
 
+func TestTruncateDecimalString(t *testing.T) {
+	tests := []struct {
+		name          string
+		input         string
+		decimalPlaces int
+		want          string
+		expectError   bool
+	}{
+		{
+			name:          "whole number",
+			input:         "4",
+			decimalPlaces: 2,
+			want:          "4.00",
+			expectError:   false,
+		},
+		{
+			name:          "single decimal",
+			input:         "4.1",
+			decimalPlaces: 2,
+			want:          "4.10",
+			expectError:   false,
+		},
+		{
+			name:          "one decimal place",
+			input:         "4.19",
+			decimalPlaces: 1,
+			want:          "4.1",
+			expectError:   false,
+		},
+		{
+			name:          "truncates to 2 dp",
+			input:         "0.149",
+			decimalPlaces: 2,
+			want:          "0.14",
+			expectError:   false,
+		},
+		{
+			name:          "does not round",
+			input:         "1.8599999999",
+			decimalPlaces: 2,
+			want:          "1.85",
+			expectError:   false,
+		},
+		{
+			name:          "high precision input",
+			input:         "123.456789",
+			decimalPlaces: 4,
+			want:          "123.4567",
+			expectError:   false,
+		},
+		{
+			name:          "zero",
+			input:         "0",
+			decimalPlaces: 2,
+			want:          "0.00",
+			expectError:   false,
+		},
+		{
+			name:          "invalid input string",
+			input:         "abc",
+			decimalPlaces: 2,
+			want:          "",
+			expectError:   true,
+		},
+		{
+			name:          "edge rounding case",
+			input:         "4.99999999",
+			decimalPlaces: 2,
+			want:          "4.99",
+			expectError:   false,
+		},
+		{
+			name:          "small value",
+			input:         "0.0001",
+			decimalPlaces: 2,
+			want:          "0.00",
+			expectError:   false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := TruncateDecimalString(tt.input, tt.decimalPlaces)
+
+			if tt.expectError {
+				if err == nil {
+					t.Errorf("TruncateDecimalString(%q, %d) expected error, got nil", tt.input, tt.decimalPlaces)
+				}
+				return
+			}
+
+			if err != nil {
+				t.Errorf("TruncateDecimalString(%q, %d) unexpected error: %v", tt.input, tt.decimalPlaces, err)
+				return
+			}
+
+			if got != tt.want {
+				t.Errorf("TruncateDecimalString(%q, %d) = %q, want %q", tt.input, tt.decimalPlaces, got, tt.want)
+			}
+		})
+	}
+}
+
 func TestParseAndScaleAmount(t *testing.T) {
 	tests := []struct {
 		name        string
@@ -64,6 +167,20 @@ func TestParseAndScaleAmount(t *testing.T) {
 			want:        "0",
 			expectError: false,
 		},
+		{
+			name:        "high decimals",
+			amount:      "1.85",
+			decimals:    "18",
+			want:        "1850000000000000000",
+			expectError: false,
+		},
+		{
+			name:        "6 d.p",
+			amount:      "2.32",
+			decimals:    "6",
+			want:        "2320000",
+			expectError: false,
+		},
 	}
 
 	for _, tt := range tests {