provide-module byline %{

    # helpers for shell-less boolean checks
    try %{ define-command -hidden true nop }
    try %{ define-command -hidden false fail }
    # used to store origina readonly status
    declare-option -hidden bool base_readonly false

	# Mappings

	map global "normal" "x" ": byline-drag-down<ret>"
	map global "normal" "X" ": byline-drag-up<ret>"

	# High-level selection expanding and contracting, based on selection direction

	define-command byline-drag-down %{
		evaluate-commands -itersel -no-hooks %{
			try %{
				byline-assert-selection-forwards
				byline-expand-below
			} catch %{
				byline-contract-above
			}
		}
	}

	define-command byline-drag-up %{
		evaluate-commands -itersel -no-hooks %{
			try %{
				byline-assert-selection-forwards
				byline-contract-below
			} catch %{
				byline-expand-above
			}
		}
	}

	# Assertions

	define-command -hidden byline-assert-selection-reduced %{
		# Selections on blank lines are not considered reduced
		execute-keys -draft "<a-K>^$<ret>"
		# Single-character selections are reduced
		execute-keys -draft "<a-k>\A.{,1}\z<ret>"
	}

	define-command -hidden byline-assert-selection-forwards %{
		try %{
			# If the selection is just the cursor or blank line
			# , we treat it as being in the forwards
			# direction, and can exit early
			byline-assert-selection-reduced
		} catch %{
			evaluate-commands -no-hooks -draft -save-regs 'ab' %{
    			# this "check" requires readonly to be false,
    			# so we store its original value before changing it
    			set-option buffer base_readonly %opt{readonly}
				# store current cursor pos
				set-register a %val{cursor_byte_offset}
				# force cursor forward
				execute-keys <a-:>
				# store second cursor pos
				set-register b %val{cursor_byte_offset}
				# check whether readonly is enabled
				try %{
    				%opt{base_readonly}
    				# disable temporarily
    				set-option buffer readonly false
				}
				# if the sel faces forward, the cursor hasn't moved
				# So we paste reg a. the pasted content is now selected.
				# We can use <a-k> to check whether it matches register b.
				try %{
					execute-keys %exp{"aP<a-k>%reg{b}<ret>u}
					# only re-enable readme if it was originally on
					try %{
    					%opt{base_readonly}
    					set-option buffer readonly true
					}
				} catch %{
					# clean up the pasted content if the above
					# assertion failed
					execute-keys 'u'
					# only re-enable readme if it was originally on
					try %{
    					%opt{base_readonly}
    					set-option buffer readonly true
					}
					# propogate failure to caller
					fail
				}
			}
		}
	}

	define-command -hidden byline-assert-selection-full-lines %{
		# Starts at beginning of line
		execute-keys -draft "<a-:><a-;>;<a-k>\A^<ret>"
		# Ends at end of line
		execute-keys -draft "<a-:>;<a-k>$<ret>"
	}

	# Low-level selection expanding and contracting primitives

	define-command -hidden byline-expand-above %{
		try %{
			byline-assert-selection-full-lines
			execute-keys "<a-:><a-;>%val{count}Kx"
		} catch %{
			execute-keys "x<a-:><a-;>"
			evaluate-commands -no-hooks %sh{
				if [ "$kak_count" -gt 1 ]; then
				echo "execute-keys '$((kak_count - 1))Kx'"
				fi
			}
		}
	}

	define-command -hidden byline-contract-above %{
		try %{
			byline-assert-selection-full-lines
			execute-keys "<a-:><a-;>%val{count}Jx"
		} catch %{
			try %{
				execute-keys "<a-x>"
			} catch %{
				execute-keys "x"
			}
			execute-keys "<a-:><a-;>"
			evaluate-commands -no-hooks %sh{
				if [ "$kak_count" -gt 1 ]; then
				echo "execute-keys '$((kak_count - 1))Jx'"
				fi
			}
		}
	}

	define-command -hidden byline-expand-below %{
		try %{
			byline-assert-selection-full-lines
			execute-keys "<a-:>%val{count}Jx"
		} catch %{
			execute-keys "x<a-:>"
			evaluate-commands -no-hooks %sh{
				if [ "$kak_count" -gt 1 ]; then
				echo "execute-keys '$((kak_count - 1))Jx'"
				fi
			}
		}
	}

	define-command -hidden byline-contract-below %{
		try %{
			byline-assert-selection-full-lines
			execute-keys "<a-:>%val{count}Kx"
		} catch %{
			try %{
				execute-keys "<a-x>"
			} catch %{
				execute-keys "x"
			}
			execute-keys "<a-:>"
			evaluate-commands -no-hooks %sh{
				if [ "$kak_count" -gt 1 ]; then
				echo "execute-keys '$((kak_count - 1))Kx'"
				fi
			}
		}
	}

}