From c47bcb5bd152097ef1ee464b71627f03886482da Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?L=C3=A9a=20Saviot?= <lea.saviot@numworks.com>
Date: Mon, 26 Aug 2019 16:28:13 +0200
Subject: [PATCH] [poincare/proba] Fix P(X<?) = 0 for binomial distribution

Returns 0 if p = 1, else NAN
---
 poincare/src/binomial_distribution.cpp |  7 ++++++-
 poincare/src/inv_binom.cpp             | 14 +++++++++++++-
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/poincare/src/binomial_distribution.cpp b/poincare/src/binomial_distribution.cpp
index 67099c834..9b6585484 100644
--- a/poincare/src/binomial_distribution.cpp
+++ b/poincare/src/binomial_distribution.cpp
@@ -77,7 +77,12 @@ T BinomialDistribution::CumulativeDistributiveInverseForProbability(T probabilit
   if (nIsZero && (pIsZero || pIsOne)) {
     return NAN;
   }
-
+  if (std::abs(probability) < precision) {
+    if (pIsOne) {
+      return 0;
+    }
+    return NAN;
+  }
   if (std::abs(probability - (T)1.0) < precision) {
     return n;
   }
diff --git a/poincare/src/inv_binom.cpp b/poincare/src/inv_binom.cpp
index e0237a5b2..0b5cc6e7c 100644
--- a/poincare/src/inv_binom.cpp
+++ b/poincare/src/inv_binom.cpp
@@ -72,7 +72,19 @@ Expression InvBinom::shallowReduce(ExpressionNode::ReductionContext reductionCon
     return replaceWithUndefinedInPlace();
   }
 
-  // TODO LEA if a == 0, check p ?
+  // If a == 0, check p
+  if (rationalA.isZero()) {
+    Expression p = childAtIndex(2);
+    if (p.type() != ExpressionNode::Type::Rational) {
+      return *this;
+    }
+    if (static_cast<Rational &>(p).isOne()) {
+      Expression result = Rational::Builder(0);
+      replaceWithInPlace(result);
+      return result;
+    }
+    return replaceWithUndefinedInPlace();
+  }
   // n if a == 1
   if (rationalA.isOne()) {
     replaceWithInPlace(n);