tdf#70673 Improve exponential trend line equation

Use negative Y if there is only 1 positive Y
Skip some 0 or 1 in some corner case.
Add minus sign if intercept is missing
Simplify writing of equation

Change-Id: I4b164568c87c54934a38ff2d0cd72133f7fece26
Reviewed-on: https://gerrit.libreoffice.org/19033
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>

1 files changed, 25 insertions, 36 deletions

diff --git a/chart2/source/tools/ExponentialRegressionCurveCalculator.cxx b/chart2/source/tools/ExponentialRegressionCurveCalculator.cxx
index a5cf10491ad5..d5c8ed0656af 100644
--- a/chart2/source/tools/ExponentialRegressionCurveCalculator.cxx
+++ b/chart2/source/tools/ExponentialRegressionCurveCalculator.cxx
@@ -54,13 +54,13 @@ void SAL_CALL ExponentialRegressionCurveCalculator::recalculateRegression(
     m_fSign = 1.0;
 
     size_t nMax = aValues.first.size();
-    if( nMax == 0 )
+    if( nMax <= 1 ) // at least 2 points
     {
         aValues = RegressionCalculationHelper::cleanup(
                     aXValues, aYValues,
                     RegressionCalculationHelper::isValidAndYNegative());
         nMax = aValues.first.size();
-        if( nMax == 0 )
+        if( nMax <= 1 )
         {
             ::rtl::math::setNan( & m_fLogSlope );
             ::rtl::math::setNan( & m_fLogIntercept );
@@ -157,48 +157,37 @@ OUString ExponentialRegressionCurveCalculator::ImplGetRepresentation(
     ::sal_Int32 nNumberFormatKey ) const
 {
     double fIntercept = m_fSign * exp(m_fLogIntercept);
-    double fSlope = exp(m_fLogSlope);
-    bool bHasSlope = !rtl::math::approxEqual( fSlope, 1.0 );
-    bool bHasIntercept = !rtl::math::approxEqual( fIntercept, 1.0 );
+    bool bHasSlope = !rtl::math::approxEqual( exp(m_fLogSlope), 1.0 );
+    bool bHasLogSlope = !rtl::math::approxEqual( fabs(m_fLogSlope), 1.0 );
+    bool bHasIntercept = !rtl::math::approxEqual( m_fSign*fIntercept, 1.0 ) && fIntercept != 0.0;
 
-    OUStringBuffer aBuf( "f(x) = ");
+    OUStringBuffer aBuf( "f(x) = " );
 
-    if ( fIntercept == 0.0)
+    if ( bHasIntercept )
     {
-        // underflow, a true zero is impossible
-        aBuf.append( "exp( ");
-        aBuf.append( getFormattedString( xNumFormatter, nNumberFormatKey, m_fLogIntercept) );
-        aBuf.append( (m_fLogSlope < 0.0) ? " - " : " + ");
-        aBuf.append( getFormattedString( xNumFormatter, nNumberFormatKey, fabs(m_fLogSlope)) );
-        aBuf.append( " x )");
+        aBuf.append( getFormattedString( xNumFormatter, nNumberFormatKey, fIntercept) );
+        aBuf.append( " exp( " );
     }
     else
     {
-        if (bHasIntercept)
-        {
-            aBuf.append( getFormattedString( xNumFormatter, nNumberFormatKey, fIntercept) );
-            aBuf.append( " exp( ");
-            aBuf.append( getFormattedString( xNumFormatter, nNumberFormatKey, m_fLogSlope) );
-            aBuf.append( " x )");
-        }
-        else
-        {
-            // show logarithmic output, if intercept and slope both are near one
-            // otherwise drop output of intercept, which is 1 here
-            aBuf.append( " exp( ");
-            if (!bHasSlope)
-            {
-                aBuf.append( getFormattedString( xNumFormatter, nNumberFormatKey, m_fLogIntercept) );
-                aBuf.append( (m_fLogSlope < 0.0) ? " - " : " + ");
-                aBuf.append( getFormattedString( xNumFormatter, nNumberFormatKey, fabs(m_fLogSlope)) );
-            }
-            else
-            {
-                aBuf.append( getFormattedString( xNumFormatter, nNumberFormatKey, m_fLogSlope) );
-            }
-            aBuf.append( " x )");
+       if ( m_fSign < 0.0 )
+            aBuf.append( "- " );
+       aBuf.append( "exp( " );
+       if ( fIntercept == 0.0 ||  // underflow, a true zero is impossible
+          ( !bHasSlope && m_fLogIntercept != 0.0 ) )    // show logarithmic output, if intercept and slope both are near one
+        {                                               // otherwise drop output of intercept, which is 1 here
+            aBuf.append( getFormattedString( xNumFormatter, nNumberFormatKey, m_fLogIntercept) );
+            aBuf.append( (m_fLogSlope < 0.0) ? " " : " + ");
         }
     }
+    if ( m_fLogSlope < 0.0 )
+        aBuf.append( "- ");
+    if ( bHasLogSlope )
+    {
+        aBuf.append( getFormattedString( xNumFormatter, nNumberFormatKey, fabs(m_fLogSlope)) );
+        aBuf.append( ' ' );
+    }
+    aBuf.append( "x )");
 
     return aBuf.makeStringAndClear();
 }