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Improve SubString nextind/prevind #24255

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Improve SubString nextind/prevind #24255

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d96fc89
make sure that for SubString nextind always returns a positive index …
bkamins Oct 21, 2017
545ef42
add a faster implementation of nextind and prevind with nchar argumen…
bkamins Oct 21, 2017
3116520
fix typo
bkamins Oct 21, 2017
c8ef411
avoid SubString of Substring
bkamins Oct 21, 2017
8bae599
make GenericString hold String only
bkamins Oct 21, 2017
f114c04
small patches
bkamins Oct 22, 2017
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37 changes: 35 additions & 2 deletions base/strings/types.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -100,8 +100,41 @@ function thisind(s::SubString{String}, i::Integer)
j-offset
end

nextind(s::SubString, i::Integer) = nextind(s.string, i+s.offset)-s.offset
prevind(s::SubString, i::Integer) = prevind(s.string, i+s.offset)-s.offset
# need to define nextind and prevind only for SubString{String}
# as other cases are handled by definitions for AbstractString
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@stevengj stevengj Dec 29, 2017 •
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These generic definitions for SubString{T} should be more efficient than the generic nextind for AbstractString if there is an efficient nextind for T. So, better to leave them in?

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Oh never mind, I guess we already discussed that above?:

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The PR is outdated (it was prepared before "string overhaul"), but the problem it solved is still present in the new code.

Therefore my thinking is to first decide on the desired behavior (I guess @StefanKarpinski will have an opinion here).

Next there is an issue of maximum efficiency of implementation. My experience is that at least SubString{String} should have a specialized method - as this will be most common case in practice and probably there is a room for efficiency improvement by using custom code.

But first we should have a defined contract we want the implementation to follow.

function nextind(s::SubString{String}, i::Integer)
# make sure that nonnegative value is returned
j = Int(i)
j < 1 && return 1
# the transformation below is valid if j>=0
nextind(s.string, j+s.offset)-s.offset
end

function prevind(s::SubString{String}, i::Integer)
e = endof(s)
# make sure that value not greater than endof(s) is returned
j = Int(i)
j > e && return e
# the transformation below is valid if j<=endof(s)+1
prevind(s.string, j+s.offset)-s.offset
end

function nextind(s::SubString{String}, i::Integer, nchar::Integer)
j = Int(i)
# if j < 1 the first valid value of j is the same as for j equal to 0
# and the transformation below is valid if j>=0
j < 0 && (j = 0)
nextind(s.string, j+s.offset, nchar)-s.offset
end

function prevind(s::SubString{String}, i::Integer, nchar::Integer)
e = endof(s)
j = Int(i)
# if j > endof(s) the first valid value of j is the same as for j equal to endof(s)+1
# and the transformation below is valid if j<=endof(s)+1
j > e && (j = e+1)
prevind(s.string, j+s.offset, nchar)-s.offset
end

function getindex(s::AbstractString, r::UnitRange{Int})
checkbounds(s, r) || throw(BoundsError(s, r))
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4 changes: 3 additions & 1 deletion stdlib/Test/src/Test.jl
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Expand Up @@ -1380,7 +1380,9 @@ the `AbstractString` interface, in order to ensure that functions can work
with string types besides the standard `String` type.
"""
struct GenericString <: AbstractString
string::AbstractString
string::String

GenericString(s::AbstractString) = new(String(s))
end
Base.endof(s::GenericString) = endof(s.string)
Base.next(s::GenericString, i::Int) = next(s.string, i)
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62 changes: 60 additions & 2 deletions test/strings/basic.jl
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Expand Up @@ -592,8 +592,9 @@ end
end

@testset "prevind and nextind" begin
let strs = Any["∀α>β:α+1>β", GenericString("∀α>β:α+1>β")]
for i in 1:2
let str = "∀α>β:α+1>β", strs = Any[str, GenericString(str), SubString(str, 1),
SubString(GenericString(str), 1)]
for i in 1:length(strs)
@test prevind(strs[i], 1) == 0
@test prevind(strs[i], 1, 1) == 0
@test prevind(strs[i], 2) == 1
Expand Down Expand Up @@ -653,12 +654,69 @@ end
@test nextind(strs[i], x, j) == n
end
end

let ss = SubString(strs[i], 1, 4)
@test prevind(ss, 0) < 1
@test prevind(ss, 0, 1) < 1
@test prevind(ss, 1, 2) < 1
@test prevind(ss, 0, 2) < 1
end
let ss = SubString(strs[i], 7, 10)
@test prevind(ss, 1) < 1
@test prevind(ss, 1, 1) < 1
@test prevind(ss, 0) < 1
@test prevind(ss, 0, 1) < 1
@test prevind(ss, 1, 2) < 1
@test nextind(ss, 4) > endof(ss)
@test nextind(ss, 4, 1) > endof(ss)
@test nextind(ss, 9) > endof(ss)
@test nextind(ss, 9, 1) > endof(ss)
end
end

@test prevind(strs[1], -1) == -2
@test prevind(strs[1], -1, 1) == -2

@test prevind(strs[2], -1) == 0
@test prevind(strs[2], -1, 1) == 0

@test prevind(strs[3], -1) == -2
@test prevind(strs[3], -1, 1) == -2

@test prevind(strs[4], -4) == 0
@test prevind(strs[4], -4, 1) == 0
end

let strs = Any["1234567", GenericString("1234567")]
for i in 1:length(strs)
@test prevind(SubString(strs[i], 1, 3), 10) == 3
@test prevind(SubString(strs[i], 1, 3), 10, 1) == 3
@test prevind(SubString(strs[i], 1, 3), 10, 2) == 2
@test prevind(SubString(strs[i], 1, 3), 3) == 2
@test prevind(SubString(strs[i], 1, 3), 2) == 1
@test prevind(SubString(strs[i], 1, 3), 1) == 0
@test prevind(SubString(strs[i], 1, 3), 3, 1) == 2
@test prevind(SubString(strs[i], 1, 3), 2, 1) == 1
@test prevind(SubString(strs[i], 1, 3), 1, 1) == 0
@test prevind(SubString(strs[i], 1, 3), 3, 2) == 1
@test prevind(SubString(strs[i], 1, 3), 2, 2) == 0

@test nextind(SubString(strs[i], 3), -6) == 1
@test nextind(SubString(strs[i], 3), -6, 1) == 1
@test nextind(SubString(strs[i], 3), -6, 2) == 2
@test nextind(SubString(strs[i], 3), 1) == 2
@test nextind(SubString(strs[i], 3), 2) == 3
@test nextind(SubString(strs[i], 3), 3) == 4
@test nextind(SubString(strs[i], 3), 4) == 5
@test nextind(SubString(strs[i], 3), 1, 1) == 2
@test nextind(SubString(strs[i], 3), 2, 1) == 3
@test nextind(SubString(strs[i], 3), 3, 1) == 4
@test nextind(SubString(strs[i], 3), 4, 1) == 5
@test nextind(SubString(strs[i], 3), 1, 2) == 3
@test nextind(SubString(strs[i], 3), 2, 2) == 4
@test nextind(SubString(strs[i], 3), 3, 2) == 5
@test nextind(SubString(strs[i], 3), 4, 2) == 6
end
end
end

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