From 6aedfb66e317dfce315cc7b48e88cd33219453c3 Mon Sep 17 00:00:00 2001
From: Romain Goyet <romain@goyet.com>
Date: Wed, 16 Sep 2015 19:14:11 +0200
Subject: [PATCH] [Poincare] Proper Integer approximation

---
 poincare/include/poincare/integer.h | 1 +
 poincare/src/integer.cpp            | 8 ++++----
 poincare/test/integer.cpp           | 4 ++++
 3 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/poincare/include/poincare/integer.h b/poincare/include/poincare/integer.h
index 74545560b..e8bc00045 100644
--- a/poincare/include/poincare/integer.h
+++ b/poincare/include/poincare/integer.h
@@ -19,6 +19,7 @@ class Integer : public Expression {
     const Integer operator+(const Integer &other) const;
     const Integer operator*(const Integer &other) const;
     bool operator==(const Integer &other) const;
+    virtual float approximate();
   protected:
     virtual void layout();
   private:
diff --git a/poincare/src/integer.cpp b/poincare/src/integer.cpp
index decc019b5..4a1ba45be 100644
--- a/poincare/src/integer.cpp
+++ b/poincare/src/integer.cpp
@@ -162,15 +162,15 @@ float Integer::approximate() {
   *
   * We can tell that:
   * - the sign is going to be 0 for now, we only handle positive numbers
-  * - the exponent is the length of our BigInt, in bits + 127
-  * - the mantissa is the beginning of our BigInt
+  * - the exponent is the length of our BigInt, in bits - 1 + 127;
+  * - the mantissa is the beginning of our BigInt, discarding the first bit
   */
   //bool sign = 0;
 
   native_uint_t lastDigit = m_digits[m_numberOfDigits-1];
   uint8_t numberOfBitsInLastDigit = log2(lastDigit);
 
-  uint8_t exponent = 127;
+  uint8_t exponent = 126;
   exponent += (m_numberOfDigits-1)*32;
   exponent += numberOfBitsInLastDigit;
 
@@ -184,7 +184,7 @@ float Integer::approximate() {
   uint_result = 0;
   //uint_result |= (sign << 31);
   uint_result |= (exponent << 23);
-  uint_result |= (mantissa >> (32-23) & 0x7FFFFF);
+  uint_result |= (mantissa >> (32-23-1) & 0x7FFFFF);
 
   return float_result;
 }
diff --git a/poincare/test/integer.cpp b/poincare/test/integer.cpp
index 100013fff..28c0a2fbd 100644
--- a/poincare/test/integer.cpp
+++ b/poincare/test/integer.cpp
@@ -21,3 +21,7 @@ QUIZ_CASE(poincare_integer_multiply) {
 QUIZ_CASE(poincare_integer_parse_integer) {
   assert(Integer::parseInteger("123") == Integer(123));
 }
+
+QUIZ_CASE(poincare_integer_approximate) {
+  assert(Integer::parseInteger("12345678").approximate() == 12345678.0f);
+}