Remove RevString

The `RevString` type for lazily reversed strings has been moved to the
LegacyStrings package. Fixes #22611.

Calling `reverse` on an `AbstractString` with no more specific method
now unconditionally returns a `String`.

Author: ararslan

NEWS.md

@@ -138,6 +138,9 @@ Language changes
 
   * The syntax `(x...)` for constructing a tuple is deprecated; use `(x...,)` instead (#24452).
 
+  * The `RevString` type for lazily reversed strings has been moved to the LegacyStrings
+    package ([#22611]).
+
 Breaking changes
 ----------------
 
@@ -288,6 +291,9 @@ This section lists changes that do not have deprecation warnings.
     Its return value has been removed. Use the `process_running` function
     to determine if a process has already exited.
 
+  * `reverse(::AbstractString)` now unconditionally returns a `String`. Previously it
+    returned a `RevString`, which has been removed from Base ([#23612]).
+
 Library improvements
 --------------------
 
@@ -1525,6 +1531,7 @@ Command-line option changes
 [#22572]: https://github.com/JuliaLang/julia/issues/22572
 [#22588]: https://github.com/JuliaLang/julia/issues/22588
 [#22605]: https://github.com/JuliaLang/julia/issues/22605
+[#22611]: https://github.com/JuliaLang/julia/issues/22611
 [#22666]: https://github.com/JuliaLang/julia/issues/22666
 [#22696]: https://github.com/JuliaLang/julia/issues/22696
 [#22703]: https://github.com/JuliaLang/julia/issues/22703
@@ -1580,6 +1587,7 @@ Command-line option changes
 [#23529]: https://github.com/JuliaLang/julia/issues/23529
 [#23530]: https://github.com/JuliaLang/julia/issues/23530
 [#23570]: https://github.com/JuliaLang/julia/issues/23570
+[#23612]: https://github.com/JuliaLang/julia/issues/23612
 [#23628]: https://github.com/JuliaLang/julia/issues/23628
 [#23665]: https://github.com/JuliaLang/julia/issues/23665
 [#23690]: https://github.com/JuliaLang/julia/issues/23690

base/deprecated.jl

@@ -2200,7 +2200,6 @@ end
     @deprecate chol!(x::Number, uplo) chol(x) false
 end
 
-
 # issue #16307
 @deprecate finalizer(o, f::Function) finalizer(f, o)
 # This misses other callables but they are very rare in the wild
@@ -2211,6 +2210,9 @@ end
 finalizer(f::Ptr{Void}, o::Ptr{Void}) = invoke(finalizer, Tuple{Ptr{Void}, Any}, f, o)
 finalizer(f::Ptr{Void}, o::Function) = invoke(finalizer, Tuple{Ptr{Void}, Any}, f, o)
 
+# Issue #22611
+@deprecate_moved RevString "LegacyStrings"
+
 # END 0.7 deprecations
 
 # BEGIN 1.0 deprecations

base/exports.jl

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

base/precompile.jl

@@ -581,9 +581,6 @@ 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

@@ -225,13 +225,13 @@ 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)
-    i = start(r)
+    i = endof(s)
     in_single_quotes = false
     in_double_quotes = false
     in_back_ticks = false
-    while !done(r, i)
-        c, i = next(r, i)
+    while i > 0
+        c = s[i]
+        nexti = prevind(s, i)
         if !in_single_quotes && !in_double_quotes && !in_back_ticks
             if c == c_start
                 braces += 1
@@ -245,18 +245,19 @@ function find_start_brace(s::AbstractString; c_start='(', c_end=')')
                 in_back_ticks = true
             end
         else
-            if !in_back_ticks && !in_double_quotes && c == '\'' && !done(r, i) && next(r, i)[1]!='\\'
+            if !in_back_ticks && !in_double_quotes && c == '\'' && i > 0 && s[nexti] != '\\'
                 in_single_quotes = !in_single_quotes
-            elseif !in_back_ticks && !in_single_quotes && c == '"' && !done(r, i) && next(r, i)[1]!='\\'
+            elseif !in_back_ticks && !in_single_quotes && c == '"' && i > 0 && s[nexti] != '\\'
                 in_double_quotes = !in_double_quotes
-            elseif !in_single_quotes && !in_double_quotes && c == '`' && !done(r, i) && next(r, i)[1]!='\\'
+            elseif !in_single_quotes && !in_double_quotes && c == '`' && i > 0 && s[nexti] != '\\'
                 in_back_ticks = !in_back_ticks
             end
         end
         braces == 1 && break
+        i = nexti
     end
     braces != 1 && return 0:-1, -1
-    method_name_end = reverseind(r, i)
+    method_name_end = i - 1
     startind = nextind(s, rsearch(s, non_identifier_chars, method_name_end))
     return (startind:endof(s), method_name_end)
 end

base/shell.jl

@@ -14,21 +14,20 @@ 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)
-    i = start(r)
-    c_old = nothing
-    while !done(r,i)
-        c, j = next(r,i)
+    i = endof(s)
+    c_old = '\0' # initialized to a null byte for type stability
+    while i > 0
+        c = s[i]
         if c == '\\' && c_old == ' '
-            i -= 1
+            i += 1
             break
         elseif !(c in _default_delims)
             break
         end
-        i = j
+        i = prevind(s, i)
         c_old = c
     end
-    s = s[1:end-i+1]
+    s = s[1:i]
 
     last_parse = 0:-1
     isempty(s) && return interpolate ? (Expr(:tuple,:()),last_parse) : ([],last_parse)

base/strings/basic.jl

@@ -646,3 +646,77 @@ next(r::Iterators.Reverse{<:AbstractString}, i) = (r.itr[i], prevind(r.itr, i))
 start(r::Iterators.Reverse{<:EachStringIndex}) = endof(r.itr.s)
 done(r::Iterators.Reverse{<:EachStringIndex}, i) = i < start(r.itr.s)
 next(r::Iterators.Reverse{<:EachStringIndex}, i) = (i, prevind(r.itr.s, i))
+
+"""
+    reverse(s::AbstractString) -> String
+
+Reverse a string. The result is always a `String`, regardless of the input type.
+
+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.
+
+# Examples
+```jldoctest
+julia> reverse("JuliaLang")
+"gnaLailuJ"
+
+julia> reverse("ax̂e") # combining characters can lead to surprising results
+"êxa"
+
+julia> join(reverse(collect(graphemes("ax̂e")))) # reverses graphemes
+"ex̂a"
+```
+"""
+reverse(s::AbstractString) = reverse(convert(String, s))
+
+## reverse an index i so that reverse(s)[i] == s[reverseind(s,i)]
+
+"""
+    reverseind(v, i)
+
+Given an index `i` in [`reverse(v)`](@ref), return the corresponding index in `v` so that
+`v[reverseind(v,i)] == reverse(v)[i]`. (This can be nontrivial in cases where `v` contains
+non-ASCII characters.)
+
+# Examples
+```jldoctest
+julia> r = reverse("Julia")
+"ailuJ"
+
+julia> for i in 1:length(r)
+           print(r[reverseind("Julia", i)])
+       end
+Julia
+```
+"""
+reverseind(s::AbstractString, i) = chr2ind(s, length(s) + 1 - ind2chr(reverse(s), i))
+
+"""
+    repeat(s::AbstractString, r::Integer)
+
+Repeat a string `r` times. This can equivalently be accomplished by calling [`s^r`](@ref ^).
+
+# Examples
+```jldoctest
+julia> repeat("ha", 3)
+"hahaha"
+```
+"""
+repeat(s::AbstractString, r::Integer) = repeat(convert(String, s), r)
+
+"""
+    ^(s::Union{AbstractString,Char}, n::Integer)
+
+Repeat a string or character `n` times.
+The [`repeat`](@ref) function is an alias to this operator.
+
+# Examples
+```jldoctest
+julia> "Test "^3
+"Test Test Test "
+```
+"""
+(^)(s::Union{AbstractString,Char}, r::Integer) = repeat(s, r)

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 +206,52 @@ 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=endof(s))
+    @boundscheck checkbounds(s, i)
+    isempty(c) && return i
+    j = Int(i)
+    @inbounds while j > 0
+        s[j] in c && return j
+        j = prevind(s, j)
+    end
+    return 0
 end
 
 function _rsearchindex(s, t, i)
     if isempty(t)
-        return 1 <= i <= nextind(s,endof(s)) ? i :
-               throw(BoundsError(s, i))
+        @boundscheck checkbounds(s, i)
+        return i
     end
-    t = RevString(t)
-    rs = RevString(s)
     l = endof(s)
-    t1, j2 = next(t,start(t))
+    j2 = endof(t)
+    t1 = t[j2]
     while true
-        i = rsearch(s,t1,i)
-        if i == 0 return 0 end
-        c, ii = next(rs,l-i+1)
-        j = j2; k = ii
+        i = rsearch(s, t1, i)
+        i == 0 && return 0
+        c = s[i]
+        ii = prevind(s, i)
+        j, k = j2, ii
         matched = true
-        while !done(t,j)
-            if done(rs,k)
+        while j > 1
+            if k < 1
                 matched = false
                 break
             end
-            c, k = next(rs,k)
-            d, j = next(t,j)
+            # Using `reverseind` with `prevind` in this way is like calling `nextind`
+            # on the reversed string
+            rk = reverseind(s, k)
+            c = s[rk]
+            k = prevind(s, rk)
+            rj = reverseind(t, j)
+            d = t[rj]
+            j = prevind(t, rj)
             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, k)
+        i = ii
     end
 end
 

base/strings/types.jl

@@ -1,6 +1,6 @@
 # This file is a part of Julia. License is MIT: https://julialang.org/license
 
-# SubString and RevString types
+# SubString type
 
 ## substrings reference original strings ##
 
@@ -109,48 +109,6 @@ 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
-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.
-
-# Examples
-```jldoctest
-julia> reverse("JuliaLang")
-"gnaLailuJ"
-
-julia> reverse("ax̂e") # combining characters can lead to surprising results
-"êxa"
-
-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)]
 
 """
@@ -172,7 +130,6 @@ 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
 

base/strings/util.jl

@@ -190,14 +190,10 @@ 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
-        i = j
+    i = endof(s)
+    while i > 0
+        s[i] in chars || return SubString(s, 1, i)
+        i = prevind(s, i)
     end
     SubString(s, 1, 0)
 end