gitpushMerge branch 'master' of github.com:drbaggy/perlweeklychallenge-club

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diff --git a/challenge-120/james-smith/README.md b/challenge-120/james-smith/README.md
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+++ b/challenge-120/james-smith/README.md
@@ -18,11 +18,52 @@ https://github.com/drbaggy/perlweeklychallenge-club/tree/master/challenge-119/ja
 
 ## The solution
 
+This is very similar to last weeks challenge - but instead of swapping bits 4-7 with bits 0-3 this time we are swapping even bits with odd bits - so the solution is similar...
+
+The even bits are found by bit-wise *and*ing the number with `0b01010101` 0r `0x55` (`85` decimal) and the odd bits are found by bit-wise anding with `0b10101010` or `0xaa` (`170` decimal). We then switch them by multiplying or dividing by 2 {which we do with the bit shift operators `<<` & `>>`}. We just have to add the two values together - as we know the bits don't overlap we can use `|` rather than `+`.
+
+```perl
+sub switch_bits {
+  ($_[0]&0xaa)>>1 | ($_[0]&0x55)<<1;
+}
+```
+
 # Task 2 - Clock Angle
 
 ***You are given time `$T` in the format `hh:mm`. Write a script to find the smaller angle formed by the hands of an analog clock at a given time.***
 
 ## The solution
 
+Given a time we can compute the position of the hour hand as `$hr*30+$min/2` and the minute hand as `$min*6`.
+
+We can then get the angle between the two as: `($hr*60+$min-$min*12)/2`
+
+We want to reduce this modulo `360` to map the value to the range `0` - `360`. **BUT** we note that `$min` may be odd and the value of the angle may therefore may not be a whole number. To get round this we perform the modulus (by `720`) before the divide by `2`.
+
+This gives us a number between `0` and `360`. But we need a number between `0` and `180`. To map the range `180` - `360` we can either use an IF OR we can note that if we
+do `abs(angle - 180)` we get the complementary angle. So we just subtract this from `180` giving the method below...
+
+```perl
+sub clock_angle_1_liner {
+  180-abs((60*(substr$_[0],0,2)-11*substr$_[0],3)%720/2-180);
+}
+```
+
+Compared to this (perhaps slightly more readable) method there is about a 60% performance gain [avoiding variables!]
+```perl
+sub clock_angle {
+  my($h,$m) = split /:/,shift;
+  my $t = abs($h*60-$m*11)%720/2;
+  return $t > 180 ? 360-$t : $t;
+}
+```
+
+**Note** We can gain more speed (90% faster than "simple" method and about 15% faster than the 1-liner) by removing the first `substr` but to do so we need to disable `numeric` warnings - `no warnings qw(numeric)`.
 
+```perl
+sub clock_angle_fast {
+  180-abs((60*$_[0]-11*substr$_[0],3)%720/2-180);
+}
+```
 
+(Without disabling warnings this gives `Argument "04:00" isn't numeric in multiplication (*) at ..` errors)