#######################################################################
## Compute power of a one sample binomial test (the exact computation)
#######################################################################
power.binom <- function(n, H0p, HAp, alpha = 0.05, alternative = "two.sided")
{
    if((length(n) > 1) || is.na(n) || (n < 1) || (n != round(n)))
        stop("n must be a positive integer")
    if(!missing(H0p) && (length(H0p)>1 || is.na(H0p) || H0p<0 || H0p>1))
        stop("H0p must be a single number between 0 and 1")
    if(!missing(HAp) && (length(HAp)>1 || is.na(HAp) || HAp<0 || HAp>1))
        stop("HAp must be a single number between 0 and 1")
    if((length(alpha)>1 || is.na(alpha) || alpha<0 || alpha>1))
        stop("alpha must be a single number between 0 and 1")
    CHOICES <- c("two.sided", "less", "greater")
    alternative <- CHOICES[pmatch(alternative, CHOICES)]
    if (length(alternative) > 1 || is.na(alternative))
        stop("alternative must be \"two.sided\", \"less\" or \"greater\"")
    if (alternative == "less") {
        PP <- pbinom(0:n, n, H0p)
        RR <- length(PP[PP < alpha])
        power <- sum(dbinom(0:(RR - 1), n, HAp))
    }
    else if (alternative == "greater") {
        PP <- 1 - pbinom((-1):(n - 1), n, H0p)
        RR <- length(PP[PP < alpha])
        power <- sum(dbinom((n - RR + 1):n, n, HAp))
    }
    else {
        alpha <- alpha/2
        PP <- pbinom(0:n, n, H0p)
        RR <- length(PP[PP < alpha])
        temp <- sum(dbinom(0:(RR - 1), n, HAp))
        PP <- 1 - pbinom((-1):(n - 1), n, H0p)
        RR <- length(PP[PP < alpha])
        power <- sum(dbinom((n - RR + 1):n, n, HAp))+temp
    }
    power
}