From 0f927d00acfa2d48591368a8d1bcfa6153285b1a Mon Sep 17 00:00:00 2001
From: mwoss <wos.mateusz16@gmail.com>
Date: Fri, 29 Dec 2023 01:56:26 +0100
Subject: [PATCH] Fix incorrect calculation of const precision, add tests

---
 const.go        | 12 +++++++-----
 const_test.go   | 34 ++++++++++++++++++++++++++++++++++
 decimal_test.go |  2 +-
 3 files changed, 42 insertions(+), 6 deletions(-)
 create mode 100644 const_test.go

diff --git a/const.go b/const.go
index 237846a..9417201 100644
--- a/const.go
+++ b/const.go
@@ -26,7 +26,7 @@ func newConstApproximation(value string) constApproximation {
 
 	var approximations []Decimal
 	for p := 1; p < maxPrecision; p *= 2 {
-		r := RequireFromString(strLn10[:coeffLen+p])
+		r := RequireFromString(value[:coeffLen+p])
 		approximations = append(approximations, r)
 	}
 
@@ -36,13 +36,15 @@ func newConstApproximation(value string) constApproximation {
 	}
 }
 
-// Returns the smallest approximation available that's at least as precise as the passed precision
-// i.e. Ceil[ log2(precision) ] = 1 + Floor[ log2(precision-1) ]
+// Returns the smallest approximation available that's at least as precise
+// as the passed precision (places after decimal point), i.e. Floor[ log2(precision) ] + 1
 func (c constApproximation) withPrecision(precision int32) Decimal {
 	i := 0
 
-	if precision > 1 {
-		precision--
+	// 0, 1, 2-3, 4-7, 8-15, 16-31, 32-63
+	// 0, 1, 2,    3,   4,    5,     6
+
+	if precision >= 1 {
 		i++
 	}
 
diff --git a/const_test.go b/const_test.go
new file mode 100644
index 0000000..67a27a7
--- /dev/null
+++ b/const_test.go
@@ -0,0 +1,34 @@
+package decimal
+
+import "testing"
+
+func TestConstApproximation(t *testing.T) {
+	for _, testCase := range []struct {
+		Const                 string
+		Precision             int32
+		ExpectedApproximation string
+	}{
+		{"2.3025850929940456840179914546", 0, "2"},
+		{"2.3025850929940456840179914546", 1, "2.3"},
+		{"2.3025850929940456840179914546", 3, "2.302"},
+		{"2.3025850929940456840179914546", 5, "2.302585"},
+		{"2.3025850929940456840179914546", 10, "2.302585092994045"},
+		{"2.3025850929940456840179914546", 100, "2.3025850929940456840179914546"},
+		{"2.3025850929940456840179914546", -1, "2"},
+		{"2.3025850929940456840179914546", -5, "2"},
+		{"3.14159265359", 0, "3"},
+		{"3.14159265359", 1, "3.1"},
+		{"3.14159265359", 2, "3.141"},
+		{"3.14159265359", 4, "3.1415926"},
+		{"3.14159265359", 13, "3.14159265359"},
+	} {
+		ca := newConstApproximation(testCase.Const)
+		expected, _ := NewFromString(testCase.ExpectedApproximation)
+
+		approximation := ca.withPrecision(testCase.Precision)
+
+		if approximation.Cmp(expected) != 0 {
+			t.Errorf("expected approximation %s, got %s - for const with %s precision %d", testCase.ExpectedApproximation, approximation.String(), testCase.Const, testCase.Precision)
+		}
+	}
+}
diff --git a/decimal_test.go b/decimal_test.go
index 29454b9..2df9591 100644
--- a/decimal_test.go
+++ b/decimal_test.go
@@ -2774,7 +2774,7 @@ func TestDecimal_Ln(t *testing.T) {
 		{"839101.0351", 25, "13.6400864014410013994397240"},
 		{"839101.0351094726488848490572028502", 50, "13.64008640145229044389152437468283605382056561604272"},
 		{"5023583755703750094849.03519358513093500275017501750602739169823", 25, "49.9684305274348922267409953"},
-		{"5023583755703750094849.03519358513093500275017501750602739169823", -1, "40.0"},
+		{"5023583755703750094849.03519358513093500275017501750602739169823", -1, "50.0"},
 	} {
 		d, _ := NewFromString(testCase.Dec)
 		expected, _ := NewFromString(testCase.Expected)