feat: create and test migration from LegacyDec->Dec (#24229)

math/CHANGELOG.md

@@ -36,7 +36,13 @@ Ref: https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.j
 
 ## [Unreleased]
 
-## [math/v1.5.1](https://github.com/cosmos/cosmos-sdk/releases/tag/math/v1.5.0) - 2025-03-28
+## [math/v1.5.2](https://github.com/cosmos/cosmos-sdk/releases/tag/math/v1.5.2) - 2025-03-31
+
+### Features
+
+* [#24229](https://github.com/cosmos/cosmos-sdk/pull/24229) Add `DecFromLegacyDec` migration function.
+ 
+## [math/v1.5.1](https://github.com/cosmos/cosmos-sdk/releases/tag/math/v1.5.1) - 2025-03-28
 
 * [#24185](https://github.com/cosmos/cosmos-sdk/issues/24185) Minor dependency bumps
 

math/dec_migrate.go

@@ -0,0 +1,8 @@
+package math
+
+// DecFromLegacyDec converts a LegacyDec to the Dec type using a string intermediate representation.
+//
+// This function can be used when migrating LegacyDec types to the Dec type.
+func DecFromLegacyDec(legacyDec LegacyDec) (Dec, error) {
+	return NewDecFromString(legacyDec.String())
+}

math/dec_migrate_test.go

@@ -0,0 +1,118 @@
+package math_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"cosmossdk.io/math"
+)
+
+// TestDecFromLegacyDec verifies that converting a LegacyDec to a Dec via string round-trip works as expected.
+func TestDecFromLegacyDec(t *testing.T) {
+	// Define test cases: a list of valid decimal string representations.
+	// Note: The legacy format always prints exactly 18 decimal places.
+	testCases := []struct {
+		name     string
+		inputStr string
+	}{
+		{"Zero", "0"},
+		{"One", "1"},
+		{"NegativeOne", "-1"},
+		{"IntegerWithNoDecimals", "123456789012345678"},
+		{"SimpleDecimal", "123.456"},
+		{"NegativeDecimal", "-9876.543210"},
+		{"SmallestUnit", "0.000000000000000001"}, // 10^-18
+		{"LargeNumber", "12345678901234567890.123456789012345678"},
+		{"TrailingZeros", "100.000000000000000000"},
+	}
+
+	for _, tc := range testCases {
+		// capture range variable
+		t.Run(tc.name, func(t *testing.T) {
+			// Create a LegacyDec from the test input string.
+			legacyDec, err := math.LegacyNewDecFromStr(tc.inputStr)
+			require.NoError(t, err)
+
+			// Convert using our conversion function.
+			dec, err := math.DecFromLegacyDec(legacyDec)
+			require.NoError(t, err)
+
+			// Convert directly from the input string for a canonical value.
+			expectedDec, err := math.NewDecFromString(tc.inputStr)
+			require.NoError(t, err)
+
+			// Compare the two Dec values.
+			require.True(t, dec.Equal(expectedDec))
+		})
+	}
+}
+
+func TestDecFromLegacyDecDeterministic(t *testing.T) {
+	// List of test input strings in legacy format.
+	testInputs := []string{
+		"0",
+		"1",
+		"-1",
+		"123.456",
+		"-9876.543210",
+		"0.000000000000000001",
+		"100.000000000000000000",
+		"12345678901234567890.123456789012345678",
+	}
+
+	// For each input, convert the legacy string to LegacyDec and then run conversion multiple times.
+	for _, s := range testInputs {
+		legacyDec, err := math.LegacyNewDecFromStr(s)
+		require.NoError(t, err)
+
+		// Run the conversion multiple times.
+		dec1, err := math.DecFromLegacyDec(legacyDec)
+		require.NoError(t, err)
+
+		dec2, err := math.DecFromLegacyDec(legacyDec)
+		require.NoError(t, err)
+
+		dec3, err := math.DecFromLegacyDec(legacyDec)
+		require.NoError(t, err)
+
+		require.True(t, dec1.Equal(dec2) && dec2.Equal(dec3))
+	}
+}
+
+// FuzzDecFromLegacyDec fuzzes the conversion function from LegacyDec to Dec.
+func FuzzDecFromLegacyDec(f *testing.F) {
+	// Seed the fuzzer with some valid input strings.
+	seedInputs := []string{
+		"0",
+		"1",
+		"-1",
+		"123.456",
+		"-9876.543210",
+		"0.000000000000000001",
+		"100.000000000000000000",
+		"12345678901234567890.123456789012345678",
+	}
+	for _, s := range seedInputs {
+		f.Add(s)
+	}
+
+	f.Fuzz(func(t *testing.T, inputStr string) {
+		// Attempt to create a LegacyDec from the fuzz input.
+		legacyDec, err := math.LegacyNewDecFromStr(inputStr)
+		if err != nil {
+			// Ignore inputs that do not form a valid LegacyDec.
+			return
+		}
+
+		// Convert using the conversion function.
+		dec, err := math.DecFromLegacyDec(legacyDec)
+		require.NoError(t, err)
+
+		// Convert directly from the legacy string output.
+		expectedDec, err := math.NewDecFromString(legacyDec.String())
+		require.NoError(t, err)
+
+		require.True(t, dec.Equal(expectedDec))
+	})
+}