42 lines
1.9 KiB
Text
42 lines
1.9 KiB
Text
do -- Damm Algorithm
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-- returns true if the check digit of s is correct according to the Damm algorithm,
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-- false otherwise #
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local function hasValidDammCheckDigit( s : string ) : boolean
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local operationTable <const> = { { 0, 3, 1, 7, 5, 9, 8, 6, 4, 2 } -- as per wikipedia example
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, { 7, 0, 9, 2, 1, 5, 4, 8, 6, 3 }
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, { 4, 2, 0, 6, 8, 7, 1, 3, 5, 9 }
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, { 1, 7, 5, 0, 9, 8, 3, 4, 2, 6 }
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, { 6, 1, 2, 3, 0, 4, 5, 9, 7, 8 }
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, { 3, 6, 7, 4, 2, 0, 9, 5, 8, 1 }
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, { 5, 8, 6, 9, 7, 2, 0, 1, 3, 4 }
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, { 8, 9, 4, 5, 3, 6, 2, 0, 1, 7 }
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, { 9, 4, 3, 8, 6, 1, 7, 2, 0, 5 }
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, { 2, 5, 8, 1, 4, 3, 6, 7, 9, 0 }
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}
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local interimDigit = 0
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for sPos = 1, # s do
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local nextDigit <const> = string.byte( s[ sPos ] ) - string.byte( "0" )
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if 0 <= nextDigit <= 9 then
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interimDigit = operationTable[ interimDigit + 1 ][ nextDigit + 1 ]
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else
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error( $"Invalid Damm digit: [{s[ sPos ]}]" )
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end
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end
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return interimDigit == 0
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end
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local function testDammAlgorithm( s : string, expectedResult : boolean ) : void
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local isValid <const> = hasValidDammCheckDigit( s )
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print( $"check digit of {s} is "
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.. if isValid then "valid" else "invalid" end
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.. if isValid == expectedResult then "" else " *** NOT AS EXPECTED" end
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)
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end
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-- test cases
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testDammAlgorithm( "5724", true )
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testDammAlgorithm( "5727", false )
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testDammAlgorithm( "112946", true )
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end
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