Binary Search
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Binary Search |
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This functions searches for a value in a sorted table via a binary search algorithm.
Files:wiki_insecure/users/chill/table.binsearch-0.3.lua
--[[ table.binsearch( table, value [, compval [, reversed] ] ) Searches the table through BinarySearch for the given value. If the value is found: it returns a table holding all the mathing indices (e.g. { startindice,endindice } ) endindice may be the same as startindice if only one matching indice was found If compval is given: then it must be a function that takes one value and returns a second value2, to be compared with the input value, e.g.: compvalue = function( value ) return value[1] end If reversed is set to true: then the search assumes that the table is sorted in reverse order (largest value at position 1) note when reversed is given compval must be given as well, it can be nil/_ in this case Return value: on success: a table holding matching indices (e.g. { startindice,endindice } ) on failure: nil ]]-- do -- Avoid heap allocs for performance local default_fcompval = function( value ) return value end local fcompf = function( a,b ) return a < b end local fcompr = function( a,b ) return a > b end function table.binsearch( tbl,value,fcompval,reversed ) -- Initialise functions local fcompval = fcompval or default_fcompval local fcomp = reversed and fcompr or fcompf -- Initialise numbers local iStart,iEnd,iMid = 1,#tbl,0 -- Binary Search while iStart <= iEnd do -- calculate middle iMid = math.floor( (iStart+iEnd)/2 ) -- get compare value local value2 = fcompval( tbl[iMid] ) -- get all values that match if value == value2 then local tfound,num = { iMid,iMid },iMid - 1 while value == fcompval( tbl[num] ) do -- ERROR: this may cause fail in fcompval if num is out of range and tbl[num] is nil tfound[1],num = num,num - 1 end num = iMid + 1 while value == fcompval( tbl[num] ) do -- ERROR: this may cause fail in fcompval if num is out of range and tbl[num] is nil tfound[2],num = num,num + 1 end return tfound -- keep searching elseif fcomp( value,value2 ) then iEnd = iMid - 1 else iStart = iMid + 1 end end end end -- CHILLCODE™
Test suite:
-- test: table size 0 t = {} local v = table.binsearch(t, 5); assert(v == nil) -- test: table size 1 t = {5} local v = table.binsearch(t, 4); assert(v == nil) local v = table.binsearch(t, 5); assert(v[1] == 1) local v = table.binsearch(t, 6); assert(v == nil) -- test: table size 2 t = {4,6} local v = table.binsearch(t, 3); assert(v == nil) local v = table.binsearch(t, 4); assert(v[1] == 1) local v = table.binsearch(t, 5); assert(v == nil) local v = table.binsearch(t, 6); assert(v[1] == 2) local v = table.binsearch(t, 7); assert(v == nil) -- test: typical, with duplicate t = {0,2,4,4,6,8,10} local v = table.binsearch(t, 0); assert(v[1] == 1) local v = table.binsearch(t, 10); assert(v[1] == 7) local v = table.binsearch(t, 4); assert(v[1] == 3 and v[2] == 4) local v = table.binsearch(t, 5); assert(v == nil) local v = table.binsearch(t, 11); assert(v == nil) local v = table.binsearch(t, -1); assert(v == nil) -- test: identical t = {1,1,1,1,1,1,1,1,1,1} local v = table.binsearch(t, 1); assert(v[1] == 1 and v[2] == 10) -- test: fcomp t = {10,52,34,44,86,38} local v = table.binsearch(t, 6, function(v) return v % 10 end); assert(v[1] == 5) local v = table.binsearch(t, 4, function(v) return v % 10 end); assert(v[1] == 3 and v[2] == 4) -- test: reverse t = {10,8,6,4,4,2,0} local v = table.binsearch(t, 6,_,1); assert(v[1] == 3) local v = table.binsearch(t, 4,_,1); assert(v[1] == 4 and v[2] == 5) print "DONE"
