remove RevString; efficient generic reverseind

These seem unrelated, but they're actually linked:

* If you reverse generic strings by wrapping them in `RevString` then
  then this generic `reverseind` is incorrect.

* In order to have a correct generic `reverseind` one needs to assume
  that `reverse(s)` returns a string of the same type and encoding as
  `s` with code points in reverse order; one also needs to assume that
  the code units encoding each character remain the same when reversed.
  This is a valid assumption for UTF-8, UTF-16 and (trivially) UTF-32.

Reverse string search functions are pretty messed up by this and I've
fixed them well enough to work but they may be quite inefficient for
long strings now. I'm not going to spend too much time on this since
there's other work going on to generalize and unify searching APIs.

Close #22611
Close #24613

See also: #10593 #23612 #24103

NEWS.md

@@ -308,6 +308,12 @@ This section lists changes that do not have deprecation warnings.
     `AbstractArray` types that specialized broadcasting using the old internal API will
     need to switch to the new API. ([#20740])
 
+  * The `RevString` type has been removed from the language; `reverse(::String)` returns
+    a `String` with code points (or fragments thereof) in reverse order. In general,
+    `reverse(s)` should return a string of the same type and encoding as `s` with code
+    points in reverse order; any string type overrides `reverse` to return a different
+    type of string must also override `reverseind` to compute reversed indices correctly.
+
 Library improvements
 --------------------
 
@@ -413,6 +419,11 @@ Library improvements
     The generic definition is constant time but calls `endof(s)` which may be inefficient.
     Therefore custom string types may want to define direct `ncodeunits` methods.
 
+  * `reverseind(s::AbstractString, i::Integer)` now has an efficient generic fallback, so
+    custom string types do not need to provide their own efficient defintions. The generic
+    definition relies on `ncodeunits` however, so for optimal performance you may need to
+    define a custom method for that function.
+
 Compiler/Runtime improvements
 -----------------------------
 

base/exports.jl

@@ -88,7 +88,6 @@ export
     Rational,
     Regex,
     RegexMatch,
-    RevString,
     RoundFromZero,
     RoundDown,
     RoundingMode,

base/precompile.jl

@@ -578,9 +578,6 @@ precompile(Tuple{typeof(Base.LineEdit.complete_line), Base.LineEdit.PromptState,
 precompile(Tuple{typeof(Base.LineEdit.input_string_newlines_aftercursor), Base.LineEdit.PromptState})
 precompile(Tuple{typeof(Base.LineEdit.complete_line), Base.REPL.REPLCompletionProvider, Base.LineEdit.PromptState})
 precompile(Tuple{getfield(Base, Symbol("#kw##parse")), Array{Any, 1}, typeof(Base.parse), String})
-precompile(Tuple{typeof(Base.isvalid), Base.RevString{String}, Int64})
-precompile(Tuple{typeof(Base.nextind), Base.RevString{String}, Int64})
-precompile(Tuple{typeof(Base.search), Base.RevString{String}, Array{Char, 1}, Int64})
 precompile(Tuple{typeof(Base.rsearch), String, Array{Char, 1}, Int64})
 precompile(Tuple{getfield(Base.REPLCompletions, Symbol("#kw##find_start_brace")), Array{Any, 1}, typeof(Base.REPLCompletions.find_start_brace), String})
 precompile(Tuple{typeof(Core.Inference.isbits), Tuple{Void, Void, Void}})

base/repl/REPLCompletions.jl

@@ -225,7 +225,7 @@ end
 # closed start brace from the end of the string.
 function find_start_brace(s::AbstractString; c_start='(', c_end=')')
     braces = 0
-    r = RevString(s)
+    r = reverse(s)
     i = start(r)
     in_single_quotes = false
     in_double_quotes = false
@@ -259,7 +259,7 @@ function find_start_brace(s::AbstractString; c_start='(', c_end=')')
         braces == 1 && break
     end
     braces != 1 && return 0:-1, -1
-    method_name_end = reverseind(r, i)
+    method_name_end = reverseind(s, i)
     startind = nextind(s, rsearch(s, non_identifier_chars, method_name_end))
     return (startind:endof(s), method_name_end)
 end

base/shell.jl

@@ -14,7 +14,7 @@ function shell_parse(str::AbstractString, interpolate::Bool=true;
                      special::AbstractString="")
     s = lstrip(str)
     # strips the end but respects the space when the string ends with "\\ "
-    r = RevString(s)
+    r = reverse(s)
     i = start(r)
     c_old = nothing
     while !done(r,i)

base/strings/search.jl

@@ -194,12 +194,6 @@ end
 search(s::AbstractString, t::AbstractString, i::Integer=start(s)) = _search(s, t, i)
 search(s::ByteArray, t::ByteArray, i::Integer=start(s)) = _search(s, t, i)
 
-function rsearch(s::AbstractString, c::Chars)
-    j = search(RevString(s), c)
-    j == 0 && return 0
-    endof(s)-j+1
-end
-
 """
     rsearch(s::AbstractString, chars::Chars, [start::Integer])
 
@@ -212,44 +206,50 @@ julia> rsearch("aaabbb","b")
 6:6
 ```
 """
-function rsearch(s::AbstractString, c::Chars, i::Integer)
-    e = endof(s)
-    j = search(RevString(s), c, e-i+1)
-    j == 0 && return 0
-    e-j+1
+function rsearch(s::AbstractString, c::Chars, i::Integer=start(s))
+    if i < 1
+        return i == 0 ? 0 : throw(BoundsError(s, i))
+    end
+    n = ncodeunits(s)
+    if i > n
+        return i == n+1 ? 0 : throw(BoundsError(s, i))
+    end
+    # r[reverseind(r,i)] == reverse(r)[i] == s[i]
+    # s[reverseind(s,j)] == reverse(s)[j] == r[j]
+    r = reverse(s)
+    j = search(r, c, reverseind(r, i))
+    j == 0 ? 0 : reverseind(s, j)
 end
 
 function _rsearchindex(s, t, i)
     if isempty(t)
-        return 1 <= i <= nextind(s,endof(s)) ? i :
+        return 1 <= i <= nextind(s, endof(s)) ? i :
                throw(BoundsError(s, i))
     end
-    t = RevString(t)
-    rs = RevString(s)
+    t = reverse(t)
+    rs = reverse(s)
     l = endof(s)
-    t1, j2 = next(t,start(t))
+    t1, j2 = next(t, start(t))
     while true
-        i = rsearch(s,t1,i)
-        if i == 0 return 0 end
-        c, ii = next(rs,l-i+1)
+        i = rsearch(s, t1, i)
+        i == 0 && return 0
+        c, ii = next(rs, reverseind(rs, i))
         j = j2; k = ii
         matched = true
-        while !done(t,j)
-            if done(rs,k)
+        while !done(t, j)
+            if done(rs, k)
                 matched = false
                 break
             end
-            c, k = next(rs,k)
-            d, j = next(t,j)
+            c, k = next(rs, k)
+            d, j = next(t, j)
             if c != d
                 matched = false
                 break
             end
         end
-        if matched
-            return nextind(s,l-k+1)
-        end
-        i = l-ii+1
+        matched && return nextind(s, reverseind(s, k))
+        i = reverseind(s, ii)
     end
 end
 

base/strings/string.jl

@@ -295,14 +295,6 @@ function first_utf8_byte(ch::Char)
     return b
 end
 
-function reverseind(s::String, i::Integer)
-    j = sizeof(s) + 1 - i
-    @inbounds while is_valid_continuation(codeunit(s, j))
-        j -= 1
-    end
-    return j
-end
-
 ## overload methods for efficiency ##
 
 isvalid(s::String, i::Integer) =
@@ -477,38 +469,6 @@ function string(a::Union{String,Char}...)
     return out
 end
 
-function reverse(s::String)
-    dat = Vector{UInt8}(s)
-    n = length(dat)
-    n <= 1 && return s
-    buf = StringVector(n)
-    out = n
-    pos = 1
-    @inbounds while out > 0
-        ch = dat[pos]
-        if ch > 0xdf
-            if ch < 0xf0
-                (out -= 3) < 0 && throw(UnicodeError(UTF_ERR_SHORT, pos, ch))
-                buf[out + 1], buf[out + 2], buf[out + 3] = ch, dat[pos + 1], dat[pos + 2]
-                pos += 3
-            else
-                (out -= 4) < 0 && throw(UnicodeError(UTF_ERR_SHORT, pos, ch))
-                buf[out+1], buf[out+2], buf[out+3], buf[out+4] = ch, dat[pos+1], dat[pos+2], dat[pos+3]
-                pos += 4
-            end
-        elseif ch > 0x7f
-            (out -= 2) < 0 && throw(UnicodeError(UTF_ERR_SHORT, pos, ch))
-            buf[out + 1], buf[out + 2] = ch, dat[pos + 1]
-            pos += 2
-        else
-            buf[out] = ch
-            out -= 1
-            pos += 1
-        end
-    end
-    String(buf)
-end
-
 function repeat(s::String, r::Integer)
     r < 0 && throw(ArgumentError("can't repeat a string $r times"))
     n = sizeof(s)

base/strings/strings.jl

@@ -1,7 +1,7 @@
 # This file is a part of Julia. License is MIT: https://julialang.org/license
 
 include("strings/errors.jl")
-include("strings/types.jl")
+include("strings/substring.jl")
 include("strings/basic.jl")
 include("strings/search.jl")
 include("strings/util.jl")

base/strings/types.jl → base/strings/substring.jl

@@ -1,9 +1,5 @@
 # This file is a part of Julia. License is MIT: https://julialang.org/license
 
-# SubString and RevString types
-
-## substrings reference original strings ##
-
 """
     SubString(s::AbstractString, i::Integer, j::Integer=endof(s))
     SubString(s::AbstractString, r::UnitRange{<:Integer})
@@ -51,6 +47,9 @@ end
 SubString(s::AbstractString) = SubString(s, 1, endof(s))
 SubString{T}(s::T) where {T<:AbstractString} = SubString{T}(s, 1, endof(s))
 
+convert(::Type{SubString{S}}, s::AbstractString) where {S<:AbstractString} =
+    SubString(convert(S, s))
+
 String(p::SubString{String}) =
     unsafe_string(pointer(p.string, p.offset+1), nextind(p, p.endof)-1)
 
@@ -123,32 +122,18 @@ function unsafe_convert(::Type{Ptr{R}}, s::SubString{String}) where R<:Union{Int
     convert(Ptr{R}, pointer(s.string)) + s.offset
 end
 
-## reversed strings without data movement ##
-
-struct RevString{T<:AbstractString} <: AbstractString
-    string::T
-end
-
-endof(s::RevString) = endof(s.string)
-length(s::RevString) = length(s.string)
-sizeof(s::RevString) = sizeof(s.string)
-
-function next(s::RevString, i::Int)
-    n = endof(s); j = n-i+1
-    (s.string[j], n-prevind(s.string,j)+1)
-end
-
 """
     reverse(s::AbstractString) -> AbstractString
 
-Reverses a string.
-
-Technically, this function reverses the codepoints in a string, and its
+Reverses a string. Technically, this function reverses the codepoints in a string and its
 main utility is for reversed-order string processing, especially for reversed
-regular-expression searches.  See also [`reverseind`](@ref) to convert indices
-in `s` to indices in `reverse(s)` and vice-versa, and [`graphemes`](@ref)
-to operate on user-visible "characters" (graphemes) rather than codepoints.
-See also [`Iterators.reverse`](@ref) for reverse-order iteration without making a copy.
+regular-expression searches. See also [`reverseind`](@ref) to convert indices in `s` to
+indices in `reverse(s)` and vice-versa, and [`graphemes`](@ref) to operate on user-visible
+"characters" (graphemes) rather than codepoints. See also [`Iterators.reverse`](@ref) for
+reverse-order iteration without making a copy. Custom string types must implement the
+`reverse` function themselves and should typically return a string with the same type
+and encoding. If they return a string with a different encoding, they must also override
+`reverseind` for that string type to satisfy `s[reverseind(s,i)] == reverse(s)[i]`.
 
 # Examples
 ```jldoctest
@@ -162,10 +147,15 @@ julia> join(reverse(collect(graphemes("ax̂e")))) # reverses graphemes
 "ex̂a"
 ```
 """
-reverse(s::AbstractString) = RevString(s)
-reverse(s::RevString) = s.string
-
-## reverse an index i so that reverse(s)[i] == s[reverseind(s,i)]
+function reverse(s::Union{String,SubString{String}})::String
+    sprint() do io
+        i, j = start(s), endof(s)
+        while i ≤ j
+            c, j = s[j], prevind(s, j)
+            write(io, c)
+        end
+    end
+end
 
 """
     reverseind(v, i)
@@ -185,10 +175,7 @@ julia> for i in 1:length(r)
 Julia
 ```
 """
-reverseind(s::AbstractString, i) = chr2ind(s, length(s) + 1 - ind2chr(reverse(s), i))
-reverseind(s::RevString, i::Integer) = endof(s) - i + 1
-reverseind(s::SubString{String}, i::Integer) =
-    reverseind(s.string, nextind(s.string, endof(s.string))-s.offset-s.endof+i-1) - s.offset
+reverseind(s::AbstractString, i::Integer) = thisind(s, ncodeunits(s)-i+1)
 
 """
     repeat(s::AbstractString, r::Integer)

base/strings/util.jl

@@ -190,16 +190,15 @@ julia> rstrip(a)
 ```
 """
 function rstrip(s::AbstractString, chars::Chars=_default_delims)
-    r = RevString(s)
-    i = start(r)
-    while !done(r,i)
-        c, j = next(r,i)
-        if !(c in chars)
-            return SubString(s, 1, endof(s)-i+1)
-        end
+    a = start(s)
+    i = endof(s)
+    while a ≤ i
+        c = s[i]
+        j = prevind(s, i)
+        c in chars || return SubString(s, 1:i)
         i = j
     end
-    SubString(s, 1, 0)
+    SubString(s, a, a-1)
 end
 
 """

stdlib/Test/src/Test.jl

@@ -1387,6 +1387,9 @@ struct GenericString <: AbstractString
 end
 Base.endof(s::GenericString) = endof(s.string)
 Base.next(s::GenericString, i::Int) = next(s.string, i)
+Base.reverse(s::GenericString) = GenericString(reverse(s.string))
+Base.reverse(s::SubString{GenericString}) =
+    GenericString(typeof(s.string)(reverse(String(s))))
 
 """
 The `GenericSet` can be used to test generic set APIs that program to

test/replcompletions.jl

@@ -3,7 +3,7 @@
 using Base.REPLCompletions
 
 let ex = quote
-        module CompletionFoo
+    module CompletionFoo
         mutable struct Test_y
             yy
         end