local fmt = require "fmt"

local function equilibrium(a)
    local len = #a
    local equi = {}
    if len == 0 then return equi end  -- sequence has no indices at all
    local rsum = a:reduce(|acc, x| -> acc + x)
    local lsum = 0
    for i = 1, len do
        rsum -= a[i]
        if rsum == lsum then equi:insert(i) end
        lsum += a[i]
    end
    return equi
end

local tests = {
    {-7, 1, 5, 2, -4, 3, 0},
    {2, 4, 6},
    {2, 9, 2},
    {1, -1, 1, -1, 1, -1, 1},
    {1},
    {}
}

print("The equilibrium indices for the following sequences are:\n")
for tests as test do
    fmt.print("%24s -> %s", fmt.swrite(test), fmt.swrite(equilibrium(test)))
end