Make startswith work with IO objects (#43055)

Co-authored-by: Steven G. Johnson <[email protected]>

NEWS.md

@@ -39,6 +39,7 @@ New library features
 Standard library changes
 ------------------------
 
+* `startswith` now supports seekable `IO` streams ([#43055])
 
 #### Package Manager
 

base/essentials.jl

@@ -946,7 +946,7 @@ function popfirst! end
     peek(stream[, T=UInt8])
 
 Read and return a value of type `T` from a stream without advancing the current position
-in the stream.
+in the stream.   See also [`startswith(stream, char_or_string)`](@ref).
 
 # Examples
 

base/strings/util.jl

@@ -67,6 +67,25 @@ function startswith(a::Union{String, SubString{String}},
     end
 end
 
+"""
+    startswith(io::IO, prefix::Union{AbstractString,Base.Chars})
+
+Check if an `IO` object starts with a prefix.  See also [`peek`](@ref).
+"""
+function Base.startswith(io::IO, prefix::Base.Chars)
+    mark(io)
+    c = read(io, Char)
+    reset(io)
+    return c in prefix
+end
+function Base.startswith(io::IO, prefix::Union{String,SubString{String}})
+    mark(io)
+    s = read(io, ncodeunits(prefix))
+    reset(io)
+    return s == codeunits(prefix)
+end
+Base.startswith(io::IO, prefix::AbstractString) = startswith(io, String(prefix))
+
 function endswith(a::Union{String, SubString{String}},
                   b::Union{String, SubString{String}})
     cub = ncodeunits(b)

test/strings/basic.jl

@@ -164,6 +164,12 @@ end
     @test endswith(y)(y)
     @test endswith(z, z)
     @test endswith(z)(z)
+    #40616 startswith for IO objects
+    let s = "JuliaLang", io = IOBuffer(s)
+        for prefix in ("Julia", "July", s^2, "Ju", 'J', 'x', ('j','J'))
+            @test startswith(io, prefix) == startswith(s, prefix)
+        end
+    end
 end
 
 @testset "SubStrings and Views" begin