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pkg-utils: describe the case conversion macros in more detail.

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After the refactoring of the case conversion macros in dd5e620ac, the
macro became even harder to understand. Improve this explanation a
little so future generations can make sense of what we're doing here.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
This commit is contained in:
Arnout Vandecappelle 2014-10-14 11:23:47 +02:00 committed by Thomas Petazzoni
parent 307df2883e
commit 9a89b98ae2
1 changed files with 17 additions and 7 deletions
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24
package/pkg-utils.mk
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@ -5,14 +5,24 @@
#
################################################################################
# UPPERCASE Macro -- transform its argument to uppercase and replace dots and
# hyphens to underscores
# Case conversion macros. This is inspired by the 'up' macro from gmsl
# (http://gmsl.sf.net). It is optimised very heavily because these macros
# are used a lot. It is about 5 times faster than forking a shell and tr.
#
# The caseconvert-helper creates a definition of the case conversion macro.
# After expansion by the outer $(eval ), the UPPERCASE macro is defined as:
# $(strip $(eval __tmp := $(1)) $(eval __tmp := $(subst a,A,$(__tmp))) ... )
# In other words, every letter is substituted one by one.
#
# The caseconvert-helper allows us to create this definition out of the
# [FROM] and [TO] lists, so we don't need to write down every substition
# manually. The uses of $ and $$ quoting are chosen in order to do as
# much expansion as possible up-front.
#
# Note that it would be possible to conceive a slightly more optimal
# implementation that avoids the use of __tmp, but that would be even
# more unreadable and is not worth the effort.
# Heavily inspired by the up macro from gmsl (http://gmsl.sf.net)
# This is approx 5 times faster than forking a shell and tr, and
# as this macro is used a lot it matters
# This works by creating translation character pairs (E.G. a:A b:B)
# and then looping though all of them running $(subst from,to,text)
[FROM] := a b c d e f g h i j k l m n o p q r s t u v w x y z - .
[TO] := A B C D E F G H I J K L M N O P Q R S T U V W X Y Z _ _