Implement indent function (#6)

* Implement indent function
* Update README.md

Co-authored-by: Nicole Epp <[email protected]>

README.md

@@ -10,7 +10,14 @@ Tooling for manipulating multiline strings.
 
 ## Features
 
-The package features a multiline string literal (`@m_str`), inspired from [YAML's block scalars](https://yaml-multiline.info/), which provide options for manipulating multiline string literals via a style and chomp indicator:
+The package features support for:
+
+- Multiline string literals (`@m_str`, `multiline`)
+- An indent function which only indents non-blank lines (`indent`)
+
+### Multiline String Literal
+
+The multiline string literal (`@m_str`), inspired from [YAML's block scalars](https://yaml-multiline.info/), which provide options for manipulating multiline string literals via a style and chomp indicator:
 
 - Style indicator:
     - `f` replace newlines with spaces (folded)
@@ -23,7 +30,7 @@ The package features a multiline string literal (`@m_str`), inspired from [YAML'
 The indicators are provided after the ending quote of the string (e.g. `m"hello\nworld!"fc`).
 If no indicators are provided the default behaviour is folded/strip.
 
-## Example
+#### Example
 
 When writing a long string literal you may want to break the string up over multiple lines in the code, to make it easier to read, but have the string be printed as a single line.
 Specifically, when writing an long error message you may want to break up the string over multiple lines:
@@ -54,3 +61,19 @@ with the non-magical computation occurring on this device.
 ```
 
 Take note that a Julia [triple-quoted string literal](https://docs.julialang.org/en/v1/manual/strings/#Triple-Quoted-String-Literals) will leave most newlines in place.
+
+### Indent
+
+The `indent` function will indent non-empty lines of a string by a number of spaces.
+
+```julia
+julia> str = """
+           A blank line:
+
+           plus another line at the end.
+           """
+"A blank line:\n\nplus another line at the end.\n"
+
+julia> indent(str, 4)
+"    A blank line:\n\n    plus another line at the end.\n"
+```

src/MultilineStrings.jl

@@ -1,6 +1,6 @@
 module MultilineStrings
 
-export @m_str, multiline
+export @m_str, indent, multiline
 
 const DEFAULT_STYLE = :folded
 const DEFAULT_CHOMP = :strip
@@ -198,4 +198,6 @@ function _process_indicators(indicators::AbstractString)
     return style, chomp
 end
 
+include("indent.jl")
+
 end

src/indent.jl

@@ -0,0 +1,41 @@
+"""
+    indent(str::AbstractString, n::Int)
+
+Indent each non-blank line by `n` spaces.
+
+# Examples
+```jldoctest; setup = :(using MultilineStrings)
+julia> indent("a\\nb", 4)
+"    a\\n    b"
+
+julia> indent("  a\\n  \\n  b", 2)
+"    a\\n  \\n    b"
+```
+
+See also `Base.unintent` and `Base.indentation`.
+"""
+function indent(str::AbstractString, n::Int)
+    n == 0 && return str
+    # Note: this loses the type of the original string
+    buf = IOBuffer(sizehint=sizeof(str))
+    indent_str = ' ' ^ n
+
+    line_start = firstindex(str)
+    blank_line = true
+    for (i, ch) in enumerate(str)
+        if ch == '\n'
+            !blank_line && print(buf, indent_str)
+            print(buf, SubString(str, line_start, i))
+            line_start = nextind(str, i)
+            blank_line = true
+        elseif blank_line && !isspace(ch)
+            blank_line = false
+        end
+    end
+
+    # Last line of string that doesn't contain a newline
+    !blank_line && print(buf, indent_str)
+    print(buf, SubString(str, line_start, lastindex(str)))
+
+    String(take!(buf))
+end

test/indent.jl

@@ -0,0 +1,8 @@
+@testset "indent" begin
+    @test indent("", 4) == ""
+    @test indent("\n", 4) == "\n"
+    @test indent("  \n  ", 2) == "  \n  "
+
+    @test indent("a", 4) == "    a"
+    @test indent("a\n\nb", 4) == "    a\n\n    b"
+end

test/runtests.jl

@@ -1,11 +1,9 @@
 using Documenter: doctest
 using MultilineStrings
-using MultilineStrings: MultilineStrings, @m_str, multiline, interpolate
+using MultilineStrings: MultilineStrings, @m_str, indent, interpolate, multiline
 using Test
 using YAML: YAML
 
-indent(str, n) = join(map(line -> (" " ^ n) * line, split(str, '\n')), '\n')
-
 function yaml_block(str, block_scalar)
     yaml = "example: $block_scalar\n$(indent(str, 2))"
     YAML.load(yaml)["example"]
@@ -28,8 +26,6 @@ const TEST_STRINGS = [
     "starting newline" => "\nbar",
 ]
 
-doctest(MultilineStrings)
-
 # Validate `yaml_block` function
 for (test, str) in TEST_STRINGS
     @assert yaml_block(str, "|+") == str
@@ -147,4 +143,8 @@ end
             @test m"""$(join(("a", "b") .* "\n", ""))"""fc == "a b\n"
         end
     end
+
+    include("indent.jl")
+
+    doctest(MultilineStrings)
 end