implemented intelligent slice functionality (#414)

implemented intelligent slice functionality

Author: awvwgk

doc/specs/stdlib_string_type.md

@@ -1254,7 +1254,7 @@ The result is a scalar `string_type` value.
 
 ```fortran
 program demo_to_title
-  use stdlib_string_type, only: string_type, to_title
+  use stdlib_string_type
   implicit none
   type(string_type) :: string, titlecase_string
 
@@ -1302,7 +1302,7 @@ The result is a scalar `string_type` value.
 
 ```fortran
 program demo_to_sentence
-  use stdlib_string_type, only: string_type, to_sentence
+  use stdlib_string_type
   implicit none
   type(string_type) :: string, sentencecase_string
 

doc/specs/stdlib_strings.md

@@ -192,3 +192,81 @@ program demo
   print'(a)', ends_with("pattern", "pat")  ! F
 end program demo
 ```
+
+
+<!-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -->
+### `slice`
+
+#### Description
+
+Extracts the characters from the defined region of the input string by taking strides.
+
+Deduction Process:
+Function first automatically deduces the optional arguments that are not provided by the user.  
+This process is independent of both input `string` and permitted indexes of Fortran.  
+Deduced `first` and `last` argument take +infinity or -infinity value whereas deduced `stride` argument takes +1 or -1 value.
+
+Validation Process:
+Argument `first` and `last` defines this region for extraction by function `slice`.  
+If the defined region is invalid i.e. region contains atleast one invalid index, `first` and 
+`last` are converted to first and last valid indexes in this defined region respectively, 
+if no valid index exists in this region an empty string is returned.  
+`stride` can attain both negative or positive values but when the only invalid value 
+0 is given, it is converted to 1.
+
+Extraction Process:
+After all this, extraction starts from `first` index and takes stride of length `stride`.  
+Extraction starts only if `last` index is crossable from `first` index with stride `stride` 
+and remains active until `last` index is crossed.  
+
+#### Syntax
+
+`string = [[stdlib_strings(module):slice(interface)]] (string, first, last, stride)`
+
+#### Status
+
+Experimental
+
+#### Class
+
+Pure function.
+
+#### Argument
+
+- `string`: Character scalar or [[stdlib_string_type(module):string_type(type)]]
+  This argument is intent(in).
+- `first`: integer
+  This argument is intent(in) and optional.
+- `last`: integer
+  This argument is intent(in) and optional.
+- `stride`: integer
+  This argument is intent(in) and optional.
+
+#### Result value
+
+The result is of the same type as `string`.
+
+#### Example
+
+```fortran
+program demo_slice
+  use stdlib_string_type
+  use stdlib_strings, only : slice
+  implicit none
+  type(string_type) :: string
+  character(len=10) :: char
+
+  string = "abcdefghij"
+  ! string <-- "abcdefghij"
+
+  char = "abcdefghij"
+  ! char <-- "abcdefghij"
+
+  print'(a)', slice("abcdefghij", 2, 6, 2)   ! "bdf"
+  print'(a)', slice(char, 2, 6, 2)           ! "bdf"
+  
+  string = slice(string, 2, 6, 2)
+  ! string <-- "bdf"
+
+end program demo_slice
+```

src/Makefile.manual

@@ -125,6 +125,8 @@ stdlib_stats_var.o: \
 stdlib_stats_distribution_PRNG.o: \
         stdlib_kinds.o \
 	stdlib_error.o
-stdlib_string_type.o: stdlib_ascii.o stdlib_kinds.o
-stdlib_strings.o: stdlib_ascii.o stdlib_string_type.o
+stdlib_string_type.o: stdlib_ascii.o \
+	stdlib_kinds.o
+stdlib_strings.o: stdlib_ascii.o \
+	stdlib_string_type.o
 stdlib_math.o: stdlib_kinds.o

src/stdlib_strings.f90

@@ -11,6 +11,7 @@ module stdlib_strings
 
     public :: strip, chomp
     public :: starts_with, ends_with
+    public :: slice
 
 
     !> Remove leading and trailing whitespace characters.
@@ -57,6 +58,14 @@ module stdlib_strings
         module procedure :: ends_with_char_string
         module procedure :: ends_with_char_char
     end interface ends_with
+    
+    !> Extracts characters from the input string to return a new string
+    !> 
+    !> Version: experimental
+    interface slice
+        module procedure :: slice_string
+        module procedure :: slice_char
+    end interface slice
 
 
 contains
@@ -290,5 +299,72 @@ elemental function ends_with_string_string(string, substring) result(match)
 
     end function ends_with_string_string
 
+    !> Extract the characters from the region between 'first' and 'last' index (both inclusive)
+    !> of the input 'string' by taking strides of length 'stride'
+    !> Returns a new string
+    elemental function slice_string(string, first, last, stride) result(sliced_string)
+        type(string_type), intent(in) :: string
+        integer, intent(in), optional :: first, last, stride
+        type(string_type) :: sliced_string
+
+        sliced_string = string_type(slice(char(string), first, last, stride))
+
+    end function slice_string
+
+    !> Extract the characters from the region between 'first' and 'last' index (both inclusive)
+    !> of the input 'string' by taking strides of length 'stride'
+    !> Returns a new string
+    pure function slice_char(string, first, last, stride) result(sliced_string)
+        character(len=*), intent(in) :: string
+        integer, intent(in), optional :: first, last, stride
+        integer :: first_index, last_index, stride_vector, strides_taken, length_string, i, j
+        character(len=:), allocatable :: sliced_string
+        length_string = len(string)
+
+        first_index = 0                 ! first_index = -infinity
+        last_index = length_string + 1  ! last_index = +infinity
+        stride_vector = 1
+
+        if (present(stride)) then
+            if (stride /= 0) then
+                if (stride < 0) then
+                    first_index = length_string + 1     ! first_index = +infinity
+                    last_index = 0                      ! last_index = -infinity
+                end if
+                stride_vector = stride
+            end if
+        else
+            if (present(first) .and. present(last)) then
+                if (last < first) then
+                    stride_vector = -1
+                end if
+            end if
+        end if
+
+        if (present(first)) then
+            first_index =  first
+        end if
+        if (present(last)) then
+            last_index = last
+        end if
+        
+        if (stride_vector > 0) then
+            first_index = max(first_index, 1)
+            last_index = min(last_index, length_string)
+        else
+            first_index = min(first_index, length_string)
+            last_index = max(last_index, 1)
+        end if
+        
+        strides_taken = floor( real(last_index - first_index)/real(stride_vector) )
+        allocate(character(len=max(0, strides_taken + 1)) :: sliced_string)
+        
+        j = 1
+        do i = first_index, last_index, stride_vector
+            sliced_string(j:j) = string(i:i)
+            j = j + 1
+        end do
+    end function slice_char
+
 
 end module stdlib_strings