feat(mobile): add send max amount helper (#84) (#477)

Co-authored-by: root <root@vmi3321457.contaboserver.net>

Author: GBOYEE

app/send.tsx

@@ -27,7 +27,7 @@ import {
   validateMemo,
 } from "../src/utils/validation";
 import { resolveAddressLabel } from "../src/utils/contacts";
-import { formatAmount } from "../src/utils/amount";
+import { formatAmount, getMaxSendableAmount } from "../src/utils/amount";
 import { WALLET_SECRET_ACCESS_MESSAGE } from "../src/utils/walletStorageErrors";
 import {
   Send as SendIcon,
@@ -140,6 +140,16 @@ export default function SendScreen() {
   const handleScanClose = () => {
     setIsScanning(false);
   };
+
+  const handleSetMaxAmount = () => {
+    const maxAmount = getMaxSendableAmount(balance);
+    setAmount(maxAmount);
+    setErrors((prev) => ({
+      ...prev,
+      amount: validateAmount(maxAmount, balance) ?? undefined,
+    }));
+  };
+
   const handleSend = () => {
     const fieldErrors: FieldErrors = {
       destination: validateAddress(destination, publicKey) ?? undefined,
@@ -245,6 +255,18 @@ export default function SendScreen() {
             error={errors.amount}
             keyboardType="decimal-pad"
             helperText={`Available balance: ${formatAmount(balance)} XLM`}
+            rightIcon={
+              <TouchableOpacity
+                onPress={handleSetMaxAmount}
+                accessibilityLabel="Send maximum amount"
+                accessibilityRole="button"
+                hitSlop={{ top: 10, bottom: 10, left: 10, right: 10 }}
+              >
+                <Text style={{ color: colors.primary, fontWeight: "600", fontSize: 13 }}>
+                  Send Max
+                </Text>
+              </TouchableOpacity>
+            }
           />
 
           <FormField

src/utils/amount.test.ts

@@ -1,4 +1,38 @@
-import { formatAmount } from './amount';
+import { formatAmount, getMaxSendableAmount } from './amount';
+
+describe('getMaxSendableAmount', () => {
+  it('returns 0 for null/undefined/empty balance', () => {
+    expect(getMaxSendableAmount(null)).toBe('0');
+    expect(getMaxSendableAmount(undefined)).toBe('0');
+    expect(getMaxSendableAmount('')).toBe('0');
+  });
+
+  it('returns 0 for zero or negative balance', () => {
+    expect(getMaxSendableAmount('0')).toBe('0');
+    expect(getMaxSendableAmount('-10')).toBe('0');
+  });
+
+  it('subtracts the default MIN_XLM_RESERVE (1) from the balance', () => {
+    expect(getMaxSendableAmount('100')).toBe('99');
+    expect(getMaxSendableAmount('1.5')).toBe('0.5');
+    expect(getMaxSendableAmount('1')).toBe('0');
+  });
+
+  it('returns 0 when balance is less than reserve', () => {
+    expect(getMaxSendableAmount('0.5')).toBe('0');
+    expect(getMaxSendableAmount('0.0000001')).toBe('0');
+  });
+
+  it('accepts a custom reserve parameter', () => {
+    expect(getMaxSendableAmount('100', 2)).toBe('98');
+    expect(getMaxSendableAmount('1', 0.5)).toBe('0.5');
+  });
+
+  it('handles number inputs', () => {
+    expect(getMaxSendableAmount(100)).toBe('99');
+    expect(getMaxSendableAmount(0.5)).toBe('0');
+  });
+});
 
 describe('formatAmount', () => {
   it('formats integers correctly', () => {

src/utils/amount.ts

@@ -1,3 +1,35 @@
+/**
+ * Minimum XLM reserve that must stay in the wallet.
+ * Accounts must keep this balance to remain active on the network.
+ */
+export const MIN_XLM_RESERVE = 1;
+
+/**
+ * Calculate the maximum amount that can be sent from a wallet,
+ * accounting for the minimum XLM reserve requirement.
+ *
+ * @param balance The current wallet balance as a string or number
+ * @param reserve Optional reserve override (defaults to MIN_XLM_RESERVE)
+ * @returns The max sendable amount as a string, or '0' if balance is insufficient
+ */
+export function getMaxSendableAmount(
+  balance: string | number | null | undefined,
+  reserve: number = MIN_XLM_RESERVE,
+): string {
+  if (balance === null || balance === undefined || balance === '') {
+    return '0';
+  }
+
+  const num = typeof balance === 'number' ? balance : parseFloat(balance);
+  if (isNaN(num) || num <= 0) {
+    return '0';
+  }
+
+  const maxSendable = Math.max(0, num - reserve);
+  // Return with up to 7 decimal places, stripping trailing zeros
+  return maxSendable.toFixed(7).replace(/\.?0+$/, '');
+}
+
 /**
  * Formats a numeric amount for display.
  * Standardises displaying XLM and other asset amounts.