Adding script to run lint check on modified lines

Bash script that uses blame on a range of commits to work out what lines
have
been touched since master. Finally it uses grep to filter the output of
the linting script based only on lines that have been modified.

The yml file that runs the linter has been updated to run  this script.

Author: thk123

.travis.yml

@@ -36,7 +36,7 @@ matrix:
       compiler: clang
       env: COMPILER=clang++
     - env: NAME="CPP-LINT"
-      script: DIFF=`git diff --name-only master HEAD` && if [ "$DIFF" != "" ]; then python scripts/cpplint.py $DIFF; fi
+      script: scripts/run_lint.sh master HEAD
 
 script:
   - make -C src minisat2-download

scripts/run_lint.sh

@@ -0,0 +1,94 @@
+#!/bin/bash
+
+set -e
+
+if [[ "$#" -ne 2 ]]
+then
+    echo "Script for running the CPP linter only on modified lines."
+    echo "Requires two arguments the start and the end"
+    echo "start - a git reference that marks the first commit whose changes to consider"
+    echo "end - a git reference that marks the last commit whose changes to consider"
+
+    exit 1
+fi
+
+git_start=$1
+git_end=$2
+
+# Get the list of files that have changed
+diff_files=`git diff --name-only $git_start $git_end`
+
+# Build a filter that will filter the blame output
+# to only include lines that come from one of the relevant_commits
+# We do this by making the blame tool output the same hash for all
+# lines that are too old.
+blame_grep_filter=`git rev-parse "$git_start"`
+
+# Build a regex for finding the line number of a given line inside blame
+# First matches the 40 digit hash of the commi
+# Then match an arbitary length number that represents the line in the original file
+# Finally matches (and groups) another arbitary length digit which is the
+# line in the final file
+regex="[0-9a-f]{40} [0-9]+ ([0-9]+)"
+
+# We only split on lines or otherwise the git  blame output is nonsense
+IFS=$'\n'
+
+are_errors=0
+
+for file in $diff_files; do
+  # We first filter only the lines that start with a commit hash
+  # Then we filter out the ones that come from the start commit
+  modified_lines=`git blame $git_start..$git_end --line-porcelain $file | grep -E "^[0-9a-f]{40}" | grep -v "$blame_grep_filter"`
+
+  if [ -z "$modified_lines" ]
+  then
+    # No modified lines in this file (perhaps only lines have been deleted)
+    # So we bail on reporting errors on this file
+    continue
+  fi
+
+  # Next we build another grep filter the output of the linting script
+  lint_grep_filter="^("
+
+  # For each modified line we find the line number
+  for line in $modified_lines; do
+
+    # Use the above regex to match the line number
+    if [[ $line =~ $regex ]]
+    then
+      # Some bash magic to get the first group from the regex (the line number)
+      LINENUM="${BASH_REMATCH[1]}"
+
+      # The format from the linting script is filepath:linenum: [error type]
+      # So we build the first bit to filter out relevant lines
+      LINE_FILTER=$file:$LINENUM
+
+      # Add the line filter on to the grep expression as we want
+      # lines that match any of the line filters
+      lint_grep_filter+=$LINE_FILTER
+      lint_grep_filter+="|"
+    fi
+  done
+
+  # Knock off the final pipe and add the closing bracket
+  lint_grep_filter=${lint_grep_filter::-1}
+  lint_grep_filter+=")"
+
+  # Run the linting script and filter by the filter we've build
+  # of all the modified lines
+  # The errors from the linter go to STDERR so must be redirected to STDOUT
+  result=`python scripts/cpplint.py $file 2>&1 | grep -E "$lint_grep_filter"`
+
+  # Providing some errors were relevant we print them out
+  if [ "$result" ]
+  then
+    are_errors=1
+    (>&2 echo "$result")
+  fi
+done
+
+unset IFS
+
+# Return an error code if errors are found
+exit $are_errors