antitrust: Tools for Antitrust Practitioners

#' @title Methods For Calculating Own and Cross-Price Elasticities
#' @name Elast-Methods
#' @docType methods
#'
#' @aliases elast-methods
#' elast
#' elast,ANY-method
#' elast,AIDS-method
#' elast,CES-method
#' elast,CESNests-method
#' elast,Linear-method
#' elast,LogLin-method
#' elast,Logit-method
#' elast,LogitNests-method
#' elast,Cournot-method
#' elast,VertBargBertLogit-method
#'
#' @description Calculate the own and cross-price elasticity between any two products in the market.
#' @param object An instance of one of the classes listed above.
#' @param preMerger If TRUE, calculates pre-merger price elasticities. If
#' FALSE, calculates post-merger price elasticities. Default is TRUE.
#' @param market If TRUE, calculates the market (aggregate) elasticity. If
#' FALSE, calculates matrix of own- and cross-price elasticities. Default is FALSE.
#'
#' @details When \sQuote{market} is FALSE, this method computes the matrix
#' of own and cross-price elasticities. Element i,j of this matrix is
#' the percentage change in the demand for good i from a small change in
#' the price of good j. When \sQuote{market} is TRUE, this method computes the
#' market (aggregate) elasticities using share-weighted prices.
#'
#' When \sQuote{preMerger} is TRUE, elasticities are
#' calculated at pre-merger equilibrium prices and shares, and when \sQuote{preMerger} is FALSE, they
#' are calculated at post-merger equilibrium prices and shares.
#'
#' @return returns a k x k matrix of own- and cross-price elasticities,
#' where k is the number of products in the market.
#'
#' @include DiversionMethods.R
#' @keywords methods
NULL

setGeneric (
  name= "elast",
  def=function(object,...){standardGeneric("elast")}
)

#'@rdname Elast-Methods
#'@export
setMethod(
  f= "elast",
  signature= "Cournot",
  definition=function(object,preMerger=TRUE,market=FALSE){

    isLinearD <- object@demand =="linear"
    slopes <- object@slopes
    intercepts <- object@intercepts


    if(preMerger){
      quantities <- object@quantityPre
      owner <- object@ownerPre
    }
    else{
      quantities <- object@quantityPost
      owner <- object@ownerPost}

    quantities[is.na(quantities)] <- 0

    quantOwner <- owner %*% quantities

    prices <- calcPrices(object,preMerger=preMerger)

    mktQuant <-  colSums(quantities,na.rm = TRUE)

    ##dPdQ
    partial <- ifelse(isLinearD,
                      slopes,
                      exp(intercepts)*slopes*mktQuant^(slopes - 1))

    ##dQdP
    partial <- 1/partial




    elast <- partial*prices/mktQuant


    if(!market){

      sharesOwner <-  t(quantOwner) / mktQuant


      elast <-  t(elast / sharesOwner)



      dimnames(elast) <- object@labels
    }

    return(elast)

  }
)


#'@rdname Elast-Methods
#'@export
setMethod(
  f= "elast",
  signature= "Linear",
  definition=function(object,preMerger=TRUE,market=FALSE){

    if(preMerger){ prices <- object@pricePre}
    else{          prices <- object@pricePost}

    slopes    <- object@slopes


    quantities <-  calcQuantities(object,preMerger)

    if(market){

      elast <-sum(slopes)/sum(quantities) * sum(quantities * prices / sum(quantities))

    }

    else{


      elast <- slopes * tcrossprod(1/quantities,prices)
      dimnames(elast) <- list(object@labels,object@labels)
    }

    return(elast)

  }
)

#'@rdname Elast-Methods
#'@export
setMethod(
  f= "elast",
  signature= "Logit",
  definition=function(object,preMerger=TRUE,market=FALSE){

    if(preMerger){ prices <- object@pricePre}
    else{          prices <- object@pricePost}


    labels <- object@labels

    alpha    <- object@slopes$alpha

    shares <-  calcShares(object,preMerger = preMerger)

    if(market){

      elast <- alpha * sum(shares/sum(shares)*prices,na.rm=TRUE) * (1 - sum(shares,na.rm=TRUE))

      names(elast) <- NULL
    }

    else{



      nprods <-  length(shares)

      elast <- -alpha  * matrix(prices*shares,ncol=nprods,nrow=nprods,byrow=TRUE)
      diag(elast) <- alpha*prices + diag(elast)

      dimnames(elast) <- list(labels,labels)
    }

    return(elast)

  }
)

#'@rdname Elast-Methods
#'@export
setMethod(
  f= "elast",
  signature= "LogLin",
  definition=function(object,preMerger=TRUE,market=FALSE){

    if(market){

      quantities <-  calcQuantities(object,preMerger)
      prices     <-  calcPrices(object,preMerger)
      elast      <-  sum(t(t(object@slopes * quantities) * 1/prices)) / sum(quantities) * sum(quantities * prices / sum(quantities))


    }

    else{
      elast    <- object@slopes
      dimnames(elast) <- list(object@labels,object@labels)
    }

    return(elast)

  }
)

#'@rdname Elast-Methods
#'@export
setMethod(
  f= "elast",
  signature= "AIDS",
  definition=function(object,preMerger=TRUE,market=FALSE){


    if(market){

      return(object@mktElast)

    }

    else{
      shares <- calcShares(object,preMerger)

      elast <- t(object@slopes/shares) + shares * (object@mktElast + 1) #Caution: returns TRANSPOSED elasticity matrix
      diag(elast) <- diag(elast) - 1
      dimnames(elast) <-  list(object@labels,object@labels)

      return(t(elast))

    }
  }
)

#'@rdname Elast-Methods
#'@export
setMethod(
  f= "elast",
  signature= "LogitNests",
  definition=function(object,preMerger=TRUE,market=FALSE){

    if(preMerger){ prices <- object@pricePre}
    else{          prices <- object@pricePost}


    nests    <- object@nests
    alpha    <- object@slopes$alpha
    sigma    <- object@slopes$sigma
    meanval  <- object@slopes$meanval

    shares <- calcShares(object,preMerger,revenue=FALSE)

    if(market){

      elast <- alpha * sum(shares*prices) * (1 - sum(shares))
      names(elast) <- NULL
    }

    else{

      sharesNests <- shares/tapply(shares,nests,sum,na.rm=TRUE)[nests]



      nprods <-  length(shares)

      elast <- diag((1/sigma-1)*alpha)
      elast <- elast[nests,nests]
      elast <- elast * matrix(sharesNests*prices,ncol=nprods,nrow=nprods,byrow=TRUE)
      elast <- -1*(elast + alpha * matrix(shares*prices,ncol=nprods,nrow=nprods,byrow=TRUE))
      diag(elast) <- diag(elast) + (1/sigma[nests])*alpha*prices

      dimnames(elast) <- list(object@labels,object@labels)

    }
    return(elast)

  }
)

#'@rdname Elast-Methods
#'@export
setMethod(
  f= "elast",
  signature= "CES",
  definition=function(object,preMerger=TRUE,market=FALSE){


    gamma    <- object@slopes$gamma

    shares_r <-  calcShares(object,preMerger,revenue=TRUE)
    shares_q <-  calcShares(object,preMerger,revenue=FALSE)

    if(market){

      if(preMerger){ prices <- object@pricePre}
      else{          prices <- object@pricePost}

      #avgPrice <- sum(prices*shares_q)/sum(shares_q)

      elast <- ( 1 - gamma  )  * (1 - sum(shares_r)) - 1

      #elast <- -1 * elast
      #elast <- -gamma * ( 1 - sum(shares_r))*avgPrice
      
      names(elast) <- NULL
    }

    else{

      nprods <-  length(shares_r)
      elast <- (gamma - 1 ) * matrix(shares_r,ncol=nprods,nrow=nprods,byrow=TRUE)
      diag(elast) <- -gamma + diag(elast)

      dimnames(elast) <- list(object@labels,object@labels)
    }
    return(elast)

  }
)

#'@rdname Elast-Methods
#'@export
setMethod(
  f= "elast",
  signature= "CESNests",
  definition=function(object,preMerger=TRUE,market=FALSE){

    nests    <- object@nests
    gamma    <- object@slopes$gamma
    sigma    <- object@slopes$sigma
    meanval  <- object@slopes$meanval

    shares <- calcShares(object,preMerger,revenue=TRUE)
    sharesNests <- shares/tapply(shares,nests,sum,na.rm=TRUE)[nests]

    if(market){

      alpha       <- object@slopes$alpha
      if(is.null(alpha)){
        stop("'shareInside' must be between 0 and 1 to  calculate Market Elasticity")}
      elast <- (1+alpha) * (1-gamma) * sum(shares) * (1 - sum(shares))
      names(elast) <- NULL

    }

    else{
      nprods <-  length(shares)

      elast <- diag(sigma - gamma)
      elast <- elast[nests,nests]
      elast <- elast * matrix(sharesNests,ncol=nprods,nrow=nprods,byrow=TRUE)
      elast <- elast + (gamma-1) * matrix(shares,ncol=nprods,nrow=nprods,byrow=TRUE)
      diag(elast) <- diag(elast) - sigma[nests]

      dimnames(elast) <- list(object@labels,object@labels)
    }
    return(elast)

  }
)


#'@rdname Elast-Methods
#'@export
setMethod(
  f= "elast",
  signature= "VertBargBertLogit",
  definition=function(object,preMerger=TRUE,market=FALSE){
  
    
  result <- elast(object@down,preMerger=preMerger,market=market)
  
  return(result)
  })