#!/usr/local/wish

canvas .c
pack .c

# I need two associative arrays here so that I can quickly
# find the item from the inder or the index from the item
# number.
#
# Note that nhigh needs to be incremented as more nodes are added,
# and both the m and n arrays need to be maintained.

set n(0) [.c create oval 10 10 20 20 -fill white -tag nodes]
set m($n(0)) 0
set n(1) [.c create oval 30 30 40 40 -fill white -tag nodes]
set m($n(1)) 1
set n(2) [.c create oval 60 60 70 70 -fill white -tag nodes]
set m($n(2)) 2
set n(3) [.c create oval 70 0 80 10 -fill white -tag nodes]
set m($n(3)) 3
set nhigh 3

for {set coords {};set i 0} {$i <= $nhigh} {incr i} {
    foreach {x1 y1 x2 y2} [.c coords $n($i)] {
	append coords "[expr $x1+(($x2-$x1)/2)] [expr $y1+(($y2-$y1)/2)] "
    }
}
set outline [eval ".c create polygon $coords -fill {} -outline black"]
.c lower $outline

.c bind nodes <2> {startdrag [.c find closest [.c canvasx %x] [.c canvasy %y]]}
.c bind nodes <B2-Motion> {movedrag %x %y}

proc startdrag {i} {
    global moveitem m n nhigh x y
    set moveitem $i
}

proc movedrag {cx cy} {
    global moveitem m n nhigh x y coords outline

    set cx [.c canvasx $cx]
    set cy [.c canvasy $cy]

    set beg [expr $m($moveitem)*2]
    set end [expr $m($moveitem)*2+1]
    set dx [expr $cx - [lindex $coords $beg]]
    set dy [expr $cy - [lindex $coords $end]]
    .c move $moveitem $dx $dy

    set coords [lreplace $coords $beg $end $cx $cy]
    eval ".c coords $outline $coords"
}