replace String length with indices

use indexing operator instead of get call
added tests

app/src/main/kotlin/net/primal/android/wallet/utils/LnInvoiceUtils.kt

@@ -72,7 +72,7 @@ object LnInvoiceUtils {
     private fun decodeUnlimitedLength(invoice: String): Boolean {
         var lower = false
         var upper = false
-        for (i in 0 until invoice.length) {
+        for (i in invoice.indices) {
             val c = invoice[i]
             if (c.code < 33 || c.code > 126) {
                 throw AddressFormatException(
@@ -95,12 +95,12 @@ object LnInvoiceUtils {
         val values = ByteArray(dataPartLength)
         for (i in 0 until dataPartLength) {
             val c = invoice[i + pos + 1]
-            if (CHARSET_REV.get(c.code).toInt() == -1) {
+            if (CHARSET_REV[c.code].toInt() == -1) {
                 throw AddressFormatException(
                     "Invalid character: " + c + ", pos: " + (i + pos + 1),
                 )
             }
-            values[i] = CHARSET_REV.get(c.code)
+            values[i] = CHARSET_REV[c.code]
         }
         val hrp = invoice.substring(0, pos).lowercase(Locale.ROOT)
         if (!verifyChecksum(hrp, values)) throw AddressFormatException("Invalid Checksum")

app/src/test/kotlin/net/primal/android/wallet/utils/LnInvoiceUtilsTest.kt

@@ -0,0 +1,60 @@
+package net.primal.android.wallet.utils
+
+import io.kotest.matchers.shouldBe
+import org.junit.Test
+
+class LnInvoiceUtilsTest {
+
+    private val validLnInvoice = "lnbc123450n1pj7welppp53umfyxp6jn9uvkt463hydtq2zpfvz78hxhpfv9wqx6v4uwdw2rnqdzqg3hkuct5v56zu" +
+    "3n4dcsxgmmwv96xjmmwyp6x7gzqgpc8ymmrv4h8ghmrwfuhqar0cqzpgxqrrsssp5tntqjpngx6l8y9va9tzd7fmtemtyp5vvsqphw8f8yqjjr" +
+    "r26x5qs9qyyssqyyv7tqp5kpsmv6s5825kcq8fxsn4ag2h5uj2j6lnsnclyyq6844khayzqrl7yue46nwlukfr4uftqcwzxzh8krqg9rqsg9tg6x" +
+    "ggszcp0gyjcd"
+
+    private val invoiceInvalidCharacter = "lnbc123450n1pj7welpp 53umfyxp6jn9uvkt463hydtq2zpfvz78hxhpfv9wqx6v4uwdw2rnqdzqg3hkuct5v56zu" +
+        "3n4dcsxgmmwv96xjmmwyp6x7gzqgpc8ymmrv4h8ghmrwfuhqar0cqzpgxqrrsssp5tntqjpngx6l8y9va9tzd7fmtemtyp5vvsqphw8f8yqjjr" +
+        "r26x5qs9qyyssqyyv7tqp5kpsmv6s5825kcq8fxsn4ag2h5uj2j6lnsnclyyq6844khayzqrl7yue46nwlukfr4uftqcwzxzh8krqg9rqsg9tg6x" +
+        "ggszcp0gyjcd"
+
+    private val invalidLnInvoice = "lnbc123450n1pj7welppp53umfyxp6jn9uvkt463hydtq2zpfvz78hxhpfv9wqx6v4uwdw2rnqdzqg3hkuct5v56zu" +
+        "3n4dcsxgmmwv96xjmmwyp6x7gzqgpc8ymmrv4h8ghmrwXuhqar0cqzpgxqrrsssp5tntqjpngx6l8y9va9tzd7fmtemtyp5vvsqphw8f8yqjjr" +
+        "r26x5qs9qyyssqyyv7tqp5kpsmv6s5825kcq8fxsn4ag2h5uj2j6lnsnclyyq6844khayzqrl7yue46nwlukfr4uftqcwzxzh8krqg9rqsg9tg6x" +
+        "ggszcp0gyjcd"
+
+    private val validLnInvoiceNoAmount = "lnbc1pvjluezpp5qqqsyqcyq5rqwzqfqqqsyqcyq5rqwzqfqqqsyqcyq5rqwzqfqy" +
+        "pqdpl2pkx2ctnv5sxxmmwwd5kgetjypeh2ursdae8g6twvus8g6rfwvs8qun0dfjkxaq8rkx3yf5tcsyz3d73gafn" +
+        "h3cax9rn449d9p5uxz9ezhhypd0elx87sjle52x86fux2ypatgddc6k63n7erqz25le42c4u4ecky03ylcqca784w"
+
+    @Test
+    fun getAmountInSatsValidInvoice() {
+        val expectedValue = 12345
+
+        val amount = LnInvoiceUtils.getAmountInSats(validLnInvoice)
+
+        amount.toInt() shouldBe expectedValue
+
+    }
+
+    @Test
+    fun getAmountInSatsValidInvoiceNoAmount() {
+        val expectedValue = 0
+
+        val amount = LnInvoiceUtils.getAmountInSats(validLnInvoiceNoAmount)
+
+        amount.toInt() shouldBe expectedValue
+
+    }
+
+    @Test(expected = IllegalArgumentException::class)
+    fun getAmountInSatsInvalidInvoice() {
+
+        LnInvoiceUtils.getAmountInSats(invalidLnInvoice)
+
+    }
+
+    @Test(expected = IllegalArgumentException::class)
+    fun getAmountInSatsInvalidCharacter() {
+
+        LnInvoiceUtils.getAmountInSats(invoiceInvalidCharacter)
+
+    }
+}